toxic family member

So I have an older sister and an older brother, I am the baby. My sister has 2 sons(6,3) and my sister I believe is a toxic member of the family. She is always back and forth between guys and is always making her situation worse than yours is. She choses to not get help for her bipolar disorder, and it cause a strain on our entire family. She is always talking about how terrible me and my brother and our spouses are and how we need to learn to live life without our parents input, but yet she doesn't have a job and my parents provide her with free child care, gas for her car and anything else she needs plus the boys are at my parents house 90% of the time. If they stop doing these things for her or bring up that she needs to get help for her mental state or try to learn to take care of herself she threatens to take the boys from us and move away and we will never see them again. And if me and my brother try to talk to her about things she makes my parents life a living hell for weeks saying they have never done anything for her and my dad doesn't really love her or her children because he is her step dad(he adopted her when she was 1) And if me and her get into a disagreement my mom always takes up for her so she won't take the kids from us and my dad will take up for me but then that cause a fight with my parents.. She is the type of person we can't even leave in the house unsupervised because we don't know if she will steal or not! and she is only nice to me and my brother or our spouses when we get our paychecks. How are me and my brother and our spouses supposed to survive in a family like this?? HELP ME! *Edit: she is 32 years old