There are at least two possibilities; First, it is not at all uncommon for a person with MS to suffer from mood swings, depression, and feelings of anxiety, anger, even rage. Sometimes it is misdirected at the people who love them the most and who they depend on for help. If I were seeing your family, I’d want to know more about how your sister is handling her diagnosis and whether she has been started on some antidepressants. Many doctors believe that the earlier one starts antidepressant medication, the better. Your sister would also probably benefit from an MS support group where people understand and support each other. People with MS can and do have productive, successful lives. Please do some research and educate yourself and your family about the usual symptoms of MS and how best to be supportive.
Then again — it does sound to me like the rest of the family is inadvertently helping your sister be a tyrant. No one is making your mom take her to a fancy restaurant. If the local diner is what she can afford, then the local diner it should be. Your sister doesn’t have to go along. If your sister hates where she lives so much, your mom can help her pack. If you don’t like the way she talks at dinner, you could just all take your plates and quietly leave the table. I have to wonder if your mom feels sorry for her or feels in some way guilty about her diagnosis. She, and the rest of you, are letting her be a bully. I suggest that instead you tell her that you love her, you wish she didn’t have MS, but that you won’t put up with her bad behavior. Then don’t.
Meanwhile, I hope your mother didn’t stop going to therapy just because your sister refused to go. I think it would be helpful for your mom, you, and your brother to see a family therapist for at least a few sessions to figure out how to be supportive of your sister, yes, but also how to work together to manage her better.
Life is short. Your mother is getting older. Your sister has an illness that isn’t going to go away. I hope you all decide to get the help you need so all family members can get to a better place with each other. You will all feel better if you do.
I wish you well.