Is there anyone who could give me some advice on how to talk to my sister about her bulimia problem?

She’s 42, lives with my mom and her son and she also has bi-polar disorder. My mother is fed up because she’s listening to my sister puke her guts up after every binge meal. she’s gone from 128 lbs to 86 lbs in the past year and getting worse. She is in therapy, but she refuses to tell her therapist about this problem, because she doesn’t see it as a problem, she tells my mom, it’s her body and stay out of her business, she’s an adult and all, but she’s also my best friend and every time I mention something about bulimia, she plays stupid and pretends she doesn’t have it. She knows the risks, she knows everything, my mom had brought home pamphlets, talked to her doctor, but the doctor told her to call the crisis hotline. He didn’t do anything. I fear for her life, she’s already had so many physical aliments that have led to surgery these past six months. I’m afraid she’s going to die right in front of us and there’s nothing we can do.