Your sister is not a person that could survive with me and my siblings. She might as well wear a sign on her head that says "Mess with me. It's fun and easy"
It's so sad that a person has to make everyone around them miserable to make themselves feel better.
The only problem is that she doesn't attempt to make ME miserable...she takes it all out on my mom. I think she knows that we'll do our best to avoid conflict because none of us (aside from her, apparently) want to cause my mom all of that stress. The way she talks about her, you'd think my mother was the absolute worst person in the world! Is she perfect? No. But she's certainly not THAT awful.
But she's also a person that you can't argue with and expect to get anywhere. Nothing will change her mind. She will always be "right" as far as she's concerned. I'm tempted to believe that she goes home, jots down what was said in the argument, and finds some way of using every statement we made against us at some later point in our lives. It's incredible the amount of effort she puts into what she does.
Now, I am convinced that there is SOMETHING wrong, and I'd like to believe that she is still getting help. I know she was at some point, but who knows now. She swings back and forth - she'll be fine for a while (treating my mother kindly), and then she'll snap. It doesn't take much. Sometimes it's just argumentative...sometimes she totally flies off of her rocker and flips out. Last time I tried to guess at what could be going on, she threatened to sue my mother and I for slander, so....who knows.
She really is a piece of work. My life is definitely much simpler when interaction with her is not required!