I'm her only daughter and have moved in to her house to look after her. She's mean to me and I do everything for her! She's hurtful but nice to my brother! I also have an autistic son who needs special care too. I'm up to my neck in anxiety and depression! I plan to move out as soon as I can.
I hate that a majority of the time I feel hopeless when dealing with my mother or when I talk to her it's hard to not be sarcastic, or I just feel sorry for her. I am so sad because I don't know her. Try to tune your mother out! With practice you can do it, in fact you may have to in order to keep your own sanity. One last thing: continue to do your best and if your brother won't help out don't become angry because anger and revenge will affect your health, not his.
Next, everyone here telling you that your responsibility is to your son is right. It is a big responsibility. There is no guilt in putting him as your second priority, you are the first.
Next, get your brother and you to her doctor's appointment and see if you can get orders to talk with a social worker about evaluating her level of care needs and then getting her set up.
Finally, that leaves you able to visit her, go to lunch or bring lunch in, and to shelter your son from whatever future decline in behaviors your mother has. If she gets mean, you leave. If she wants company, she will have to learn not to be mean.
Good luck, your hands are full right now.
I have not much to add.
If you have a son with autism who must have first claim on your attention, and a brother whose company your mother appears to prefer to your own, I slightly struggle to see what difficulty you're having with a decision, here.
"Mother, meet brother. Brother, meet mother." And leave them to it.
On the other hand: where were you living before you moved in with your mother? Is your situation a bit more complicated than you've described, perhaps?
Good luck to you and hugs!