My older sister and her best friend have known each other for almost 20 years now. We're neighbours and we grew up together. Our family long has started viewing her as a fellow family member and I always thought we got along great. A year ago my sister started really obsessing with her friend, it was around the time she got engaged and shortly after got a really good paying job and overall she's just very happy and has been on somewhat of a streak lately. My sister hasn't been so lucky as of yet. She didn't get the job she was always pursuing and she also hasn't been in a serious relationship for years. Nowadays she's having mental breakdowns with her best friends average gym or vacation posts. She literally gets physically sick, insults her 24/7 but then tries to hang out with her all-day everyday. When they're together at our house my sister is passive-agressive all time. I can only speak about what I personally witnessed and I've never experienced that her friend was rude to us or her even once. It reached its climax 2 weeks ago. Her friend officially gave us the wedding date. She came to tell us in the morning. My sister has been sick all day, like REALLY sick to the point I wanted to drive her to the hospital. I told her that she should consider just distancing herself when she thinks the relationship doesn't do her any good, but she doesn't give a proper answer to that. She's acting childish and it's really starting to mentally and physically ruin her. Tl;dr - I know she's an adult and she should take care of such childish matters herself, but I'm honestly worried about her. This isn't looking healthy. I've never seen her this aggressive and desperate. She has an unhealthy obsession with her friend and I dunno how I can help her.