What should I do about crazy sister??

My sister has had gastric byp surgery two years ago (went from a size 22 to a size 10. For the last year she has been unbearable, rude to family and friends. Disrespect her husband and is just mean to be around. We figured it would be this way for a while but then she would be her old self (kind, gererous, loving), we figured that she never got to be a so called "wild y teen" like we all had the privilage to be. Now she is out of control. She also is cheating on her husband. Family and friends can't stand to be around her and her materialistic-I'm better than everyone attitude. It makes me sad and I want my sister back. So, what should i do?? Intervention......would she feel we are attacking her? How do I approach this?