Getting financial details. How can we get details about all of her accounts and make sure they are being managed appropriately?

I believe if none of you are moms POA, your hands are tied at the moment. The fact that your mom now has dementia, means she won't be able to request someone to be her POA either, so you may have to look into getting guardianship of your mom, as that will allow you to take over everything for her. It can be costly to do so, but if you believe moms accounts are being abused in any way, it might be worth it. Best wishes.

If she is not POA then she isn't legally a fiduciary with the burden of keeping meticulous records. If she was she would not be beholden to you to discuss anything whatsoever, and in fact SHOULD NOT do so as this is private information of your Mother.
If you suspect Fraud it is time to take your evidence to APS and ask they open a case. If you have no evidence you should consider getting guardianship of your demented Mom if you wish to have it. If you do not wish to have it (YOU would then be the Fiduciary with all the burdens of that task in meticulous record keeping. You could also consider becoming the representative payee for your Mom's social security. Look on their site to see what all is involved; it's a lot.
You Mom apparently put her DIL on her accounts, or her name is there since the passing of her husband.
I think ultimately, if she is not open with you and you are worried, you are stuck with seeking guardianship.

How do you know for a fact that she didn't at some point become your mom's PoA? But if she isn't PoA, then funkygrandma59 is correct about guardianship by either the family or the county. You must pursue it through the courts and it can be costly and time-consuming and if the step-daughter contests it, the judge may decide to minimize the chaos for your mom's benefit and assign a different guardian. You can try to contact her county's social services to see if temporary emergency guardianship is a possibility, otherwise you should contact an elder law attorney who does guardianship. If the county gets guardianship then there is no transparency at all into her affairs and the guardian makes all decisions moving forward, including what facility she lives in.

This is a sticky situation when there are second marriages and each has their own children.

IMO you have as much right to know what is going on with the bank accounts as his daughter does. To be honest, he is dead so Mom is now in charge of the accounts. Daughter's ability to write checks may have stopped at his death. Did he have a Will? Are the accounts in Mom's name too?

I would see a lawyer on Mom's behalf. She is entitled to at least half of the assets. Maybe a letter from him requesting an accounting maybe enough. You need to protect what is Moms.

I'm concerned by "She is not POA, just a signer on the checking account." Is she jointly on the checking account with your mother or is she just an authorized signer for your mother? There is a big difference and her reluctance to provide any information to you all is very concerning. I think you need to talk to a lawyer and see what they suggest to get you access to information. Sometimes all it takes is a letter from the lawyer making the request to shake the information loose.

Without being your mom's POA, you have no authority to access any of her accounts. In fact, the bank may not even recognize a POA and may require a bank authorized POA. The person signing those checks doesn't have the authority either. The bank should be challenging these signatures. Is this person signing “as POA for (your mom's name)” even though she isn't POA? The bank shouldn't be accepting these checks if they don't have her POA on file. This person's actions are fraudulent. I would notify the bank and ask why are they clearing these checks and also report this financial abuse to APS.

Probably need an atty if she's not handing the info over. Basically, her dad is dead and now she's involved in financial affairs of a woman she's not really related to. However, her name was added to the banking account (POA or not). Is your mom's name on this account? Is her dad's name still on it? Atty best person to figure it out.

On the other hand, did this daughter do all the work for your mom/her dad? If you/your sibs allowed her to do the hard work then - it creates issues to call her out now. Is that why she's not entertaining your request for records?

Take mom to the bank and ask for copies of her financial statements. The bank won't care that she has dementia as long as she is present. You can probably also close the old acct and open a new one. I also called social security on mom's behalf. They spoke briefly to her to get an ok to talk to me. I followed up with sending in the representative payee form. I had to do all this because a younger brother was stealing all her money.

What proof do you have that she does not have POA? Her actions indicate, to me, that she has some type of legal authority. POA can be easily changed, unless it is irrevocable.

If she's not DPOA and not family but Signer on checks, who gave her this authority? Her father? And was the father running finances when alive and were these mostly joint, or was she mainly taken care of by him? It sounds as if your mother was set up by her late spouse akin to how he ran things when he was alive.

If you have not had any dealings with your mother's finances and these were her spouse's then he sounds like he set her up without your assistance and this sounds like "old business " I would Tread lightly if there are sizeable funds, the relative sounds like His Daughter which means your mother may not have been given total access to any funds and use of other legal entities might have been created by him to allow his wife to live in her home, get bills paid, etc., but she might not have freedom or never had this freedom to any money, if it was mostly his.

I was privy to a couple married over 40 years, wife never saw a checkbook or a bill in all of 40 years, and even at his death, he gave all accessibility for her livelihood to someone else, and she was not able to change this.

You can see what an attorney tell you, but the most you might be getting is Medical POA, if she does not have this. Otherwise, you do not know if your mother is living off late spouse income and other monies that she cannot manage and never managed and apparently spouse did not give anyone but his relative this trust possibly with more legal documentation than you know. You might not have any Rights to this information if there is nothing proving you had any relationship to this couple's finances previous to his death. It will come off as you seeking money via mom, when you need to show there was an investment other than being related. Many people look for relationship when it comes to funds but cannot show investment in one's life while living. This was setup for her protection not for others to look into what may be going on. Why interest now? If her spouse was alive, would same be asked of him? I doubt this and might be why he chose his next of kin, not his wife's. Take an interest in her well-being not her finances, it sounds better. By the way. Stepdaughter did not "take" over anything, this was given to her and apparently acknowledged by the banking institution, which means Stepdaughter is possibly named on all her accounts upon her death as well. This is all she needs and this was done while her father was alive. So, your mother cannot reverse anything now, and the banks will not honor these things due to Stepdaughter being assigned, trusted person. You have very little recourse that does not spell out, concern for what you can get, when no concern was present throughout her life, moreover now she has dementia, we need to helo her see what the Stepdaughter is doing? It sounds like mom is also okay with things and the last thing someone with dementia needs is members of their family trying to make them fear who they are dependent on. I hope everyone realize the relationship with mom had many years to grow and become trusted. You cannot step in something that really does sound like undermining at this late stage.

You will need to prove your investment has been more resilient and lifelong than the Stepdaughter's and I do not think this is the case, or this might have been done differently by her late-spouse. If money was given out to her family more than once, loans made but unpaid, hand outs given, and nothing retrieved, then make sure you are aware of all of these things, his daughter is aware of why he wanted her to do this, and doing it for your mom protection, so must ask yourself why now?

I'm assuming you are POA? The POA can contact all of the banks, credit card companies, financial institutions, etc. and ask them to send a copies of the statements (or set up online access to her accounts). But you'll need to get copies of the statements in order to know what she has and to contact them. You might need an attorney, if she is uncooperative. Are you the POA? If so, you have a right to see all of the statements and handle her finances.

If you have POA, you have every right to access your Mom’s finances, act in her best interest and handle her affairs. You can go to the bank with true copies of your Durable POA, your identification and account numbers if you have them. Your Mom’s late husband’s daughter should have relinquished that responsibility, there is no longer any bloodline. Is it possible the husband’s daughter fears once she gives up that control financially, she won’t get her “fair share” of her late father’s estate?

POA with financial authority can request this information from all financial institutions. If you suspect fraud or elder abuse regarding finances, contact the local authorities where mom lives.

Also POA for financials can close accounts or get Mom off of joint accounts.

People think caregiving should be done for free. Wow.

Contact Elder Law Attny ASAP.

Is your mom being cared for appropriately? Does she go without necessities because of her stepdaughter?

If yes and no, and you are not POA for your mom, then it is none of your business what is happening with the money.

Your step sister owns the account, there is no such thing as signatory rights with any bank that I have ever done business with. You are a joint account holder and you have as much right to the money in the account as the other joint account holder.

Obviously, her dad trusted her to take care of his wife after his death or he wouldn't have put her in control of the finances.

What is it that makes you question the situation now?

My suggestion is to find a lawyer who does elder law. Ours has been so valuable in negotiating the many bumps in the road while caring for my elder sister who has dementia. Without someone having POA, it is not likely that you will be able to determine anything about debts, income, assets etc. I wish you well. None of this is easy!

Why aren't you taking care of your own mom instead questioning this caregiver.
You want her to do it for free?

What role did you play before your  (step?) father died?    Why and/or how has that changed so that you're no longer playing a role which presumably allowed you access to the financial data?

Who is the POA?
Have you seen an elder law attorney?
What changed when your dad died that the step sister is signer on the account and when did you find out she is a signer?
Has your step sister's life style changed? Having more possessions or money to do things?
Is your mom lacking care and necessities?

I have no idea how family aloud a non POA and also a person not blood related to ever be on account with her in the first place. You should have taken steps to stop this as soon as you found out what had happened. You will need to hire an attorney and have your mother declared incompetent at once. If that attorney can prove your mother was taken advantage of because of her Dementia then she will
have to repay what she spent. Sense that person is an official signer on the account she can spend the money how ever she sees fit unless it’s proven she knew your mother was not of sound mind . She may have already signed over the house or any other assets available Good luck 😂

Heartaches, I'm troubled by one of your comments.   This is not a criticism, but it is a commentary and caution on  inaccurate advice.

You wrote:

"Sense (sic) that person is an official signer on the account she can spend the money how ever she sees fit unless it’s proven she knew your mother was not of sound mind . "

This is not true.   DPOAs and POAs, if properly prepared, specify the range of assets covered, the actions that can be taken, and from what I've seen, in enough detail to provide limitations with a given range.  

The fiduciary is not given free range outside of the needs of the primary individual.    No one can spend however he or she sees fit, regardless of mental status.   And certainly no one can lawfully spend funds on herself.

Depending on how far along mom's dementia is, sometimes if it's early enough you can get POA. Depends on how an EC atty views her cognitive ability.

For the most part, I should think the only thing that can be done, IF someone else doesn't have POA already, is seek legal help (EC atty) and file for guardianship.

Does anyone have POA? If so, that person should be stepping in NOW and taking over everything. Since the SD is likely joint on the account, you will likely have to just open another account, and w/draw all funds to deposit into the new POA account. SS would have to be diverted if that goes into that account. With dementia, you will need to sign up as Rep Payee. It is the ONLY legal way, per SS, to manage another person's SS funds. It requires a special Rep Payee account, to be set up once they approve you. First payment will be a check, then when the acct is set up, you can get electronic deposit to the acct.

Both SS and guardianship will require reporting how the funds are used. She will need care, so are any of you prepared to take on that role? Who has been caring for her since SF passed? Has SD been caring for her? Do any of you take care of her, see her, inquire about her well being? Are you local and can drop in to visit/help with her care?

Too many questions to really understand the whole picture....

Consult an elder care attorney, but it doesn't sound like you can do anything. Someone gave the stepdaughter authority to access those finances and the bank recognized it as legal.
There are a lot of harsh comments here that seem unnecessary about what your prior relationship may or may not have been with your mom. None of us has any information that qualifies us to judge that. I hope going forward you can share some quality moments with your mom. In the end those memories are what will mean the most.

Given the lack of "documentation" that she does not have, and not willing to provide information, give me the chills. I would immediately consult with an eldercare attorney (possible free one hour consultation) and ask how this should be handled. This could be a slippery situation in which you may most likely need some outside help. You could also in confidence talk to the bank and ask them what they think about the situation and how to get help. Good luck. I don't think you can do this one alone.

If could be possible that she was listed as joint owner on the account. If that’s the case, she can do whatever she wants even close out the account & open a new one.

Before anyone comments this happened to me. My mom had my sibling & myself on her accounts with her as joint owners. My sibling went in & closed the account & opened a new one without my name on it.

I contacted the bank & they told me it was perfectly legal even though it was my mom’s SS & pension checks that were going in there. They told me that any one of us could have done that.

I agree with Riley...below... But since u mention non-POA, is there someone that DOES has POA, I would definitely speak with a lawyer, who might be able to stop the step-daughter immediately (or freeze the banking accounts). Good luck.

Imho, perhaps you should retain an elder law attorney.

If the late husband's daughter has signing privileges (?) or is a joint account holder, she can do whatever she wants to with that account. You may hope your mother's money is being properly managed, but you really have no standing here in getting details about accounts on which you are not a joint account holder.