I need advice on how to respond to my sister with regard to my mother.

I described an issue I am having with my mother in some detail a few weeks ago. She had suddenly shunned me, going from normal interactions to treating me as though I were poisonous without warning and without any clear reason (she gave me a reason but it made no sense and she won't clarify or even talk to me). All those details and the background to this question are in that earlier one.
I have one sibling, a sister, who lives quite some distance away. I will call her Lori here. Lori is a truly wonderful person who leads a good life and always tries to do the right thing. However, she is one of those people whose response to anything someone says is disagreement. She does this with everyone. She is very polite about it, but seems to feel the need to correct others constantly. For instance, if I were to say that I had started walking, she would immediately respond with "Are you sure that is smart? You might catch a cold - have you considered walking in a mall or on a treadmill?" If I said I had started walking indoors she might say "hmmmmm.....you are missing out on the sunshine and beauty of the world, don't you think?".

A little history - my mom moved across the country several years ago to be near my sister's family since she was lonely and felt that my family did not spend the time with her that she needed. There is some truth to that - my mom (and sister) are both very social types who enjoy sightseeing, gossiping and shopping. I am very much a homebody and do not thrive on such things. I am disabled so it is hard to get around, but even if I weren't disabled, this is how I am. I do make friends easily with direct, up front women, but have never been adept at the social dance that my mom and sister thrive upon. My mom is generally nice but is abruptly rude, unpredictable, and generally stressful to be around for my husband and children (and me) since she quickly alternates between being extra nice to giving unwanted advice (that is often rude) and criticism masked as concern. So no, we weren't always flocking to spend time with her. So she moved to the east coast, and during the time she lived there, complained to me that she didn't know why she moved there, since she was just as lonely (Lori was home schooling her 4 kids at the time). Lori would call me now and then baffled and hurt by my mom doing the very thing she is currently doing to me (cutting off contact except for dramatic, hurtful emails). Eventually mom decided to move to my small town. She knew what she was getting into, yet bought a massive house and created a showplace for her family to enjoy. Clearly, she expected that we would gather round her hearth and become a Norman Rockwell painting. The reality however, is different. I have trouble even accessing her stairs and cannot physically handle the long walks/sightseeing/shopping trips she constantly requests. My boys are teens and she does not allow their friends to her home, and becomes insulted if they text. She always has jobs for them to do and advice on how to change their appearance, etc. She basically interviews them and does not listen when they (or anyone) tries to talk. In anyone's mid sentence she will walk off or turn to talk to someone else. So I have to cajole them into visiting and being attentive when she visits here (if their friends show up she rudely leaves).
Well after all this stuff happened last month with mom, I was initially very worried about her and since she wouldn't have anything to do with me, I called Lori to say that I wasn't sure what was going on, and wanted to ask her to be there for mom even more than usual (they speak daily) since she was clearly going through some emotional upheaval. Lori responded that mom seemed fine and unusually chipper. So I told her what had been happening (the same basic things Lori said to me a few years ago) and Lori immediately went into her polite disagreement thing. The conversation actually went in a complete circle until she was telling me what I had said in the first place - it was rather amusing to me. What I got out of the discussion however, was a feeling of defeat. Lori made it clear that my poor mom is lonely because we ignore her. So frustrating to hear that! We do endless things for her, talk daily, visit several times a week (she is independent, constantly working, and most of the time says she is too busy to do anything). She wants to do things on her terms, when she chooses, and usually at the last minute.
My sister is arriving in town today, however I am currently banished, so had to make an appointment to see her in a few days. I don't want to discuss my mom with her, but I know she will push it, if only to emphasize the opposite of whatever I say. If I say I would rather not discuss mom, she will want to know why, and will insinuate that I am indeed uncooperative. It is so frustrating, especially since I put continual effort into making mom happy and am hurt and confused by what is happening now.