Sister is upset we don't come around

My sister is offended that we don't come around anymore. I don't like going around her because she has this I'm better than you attitude and has passed it on to her daughter. Her daughter is 17 and she has pretty much raised her to think she is better and everyone is under her. Her daughter is a mean girl and a bully. She's gotten in trouble in her school for bullying and my sister refuses to punish her. Theres a difference between wanting your child to have confidence and raising them to think everyone is under her. My daughter is 14 and autistic and my niece bullies her so bad. She calls her retarded, ugly, has pushed her. The last straw was when we spent the night and my daughter wets the bed sometimes so she wear goodnight to bed. She forgot to put them on and ended up peeing my nieces bed and my niece hit her! My sister told me "Well its not my fault she's retarded. " She's a grown women calling her niece retarded where she can hear. I stopped going over. My sister got mad that I missed her daughters birthday and says my issue is my daughter is too sensitive. She was like " We are still family. You should have been there." Well I'm sick of her kids bullying! My daughter may be sensitive but that doesn't mean she deserves to be mistreated!