How do you deal with a toxic family member?

I am a young adult and one of 5 children. My older sister and I are 2.5 years apart and have always struggled with a relationship. Since she was 13, she has caused problems in our family due to her drug abuse, dysfunctional lifestyle, selfishness, etc. I have always been an enabler by taking care of her and trying to help her. She has been to therapeutic boarding school, drug treatment, etc. Recently, she has gone psycho and has started treating me worse than anyone. My parents have severe codependency and blame her lack of being able to grow up and take responsibility for herself on her ADD. I want nothing to do with her for the time being and want to set my boundaries but am having a hard time due to my close-knit family. They don't understand why I don't want to be involved with a toxic person like herself. Especially now that I am looking to get married and settle into having a family. Any ideas on how to respectful and maturely deal with this?