This is very important to me, so I need opinions, advice, thoughts. Anything.

This may be a little long, but this is very important to me. So my younger sister is 19. She was doing really great in highschool, and even got a scholarship. Once she graduated, and started dating this guy she has completely went downhill. She has been pushing everyone close to her out of her life. Even my mom and my stepdad. She moved in with him at his grandparents house a couple months back, and is engaged to him. They've been dating for about 6 or 7 months. Recently we really started noticing a change in her. Shes lost soo much weight. She literally looks like a walking skeleton. Her leggings look similar to sweatpants on her. Shes sort of pale looking, and has dark black bags under her eyes. She randomly called my mom one day and went completely off on her and was cussing her out which is completely unlike my sister. We recently found out that her fiance has been to rehab a few times for meth. We are starting to become concerned because my sister has never been this small, and has never just pushed all of us away, and never called and cussed out my mom or stepdad. She wont get a job either, or gets an interview and doesnt go to it. Or she does and then all of a sudden the job doesn't work out. I guess what I'm wondering is there anyone who knows any characteristics of someone who could be doing drugs, or anything like weight loss or the complete flip of personality? Advice on maybe how to find out? Personal opinions on what you may think? Anything? My family and I are very worried about her, and dont want anything to happen to her.