How can I help an anorexic family member?

I have a relative that used to be alittle overweight, but she started weightwatchers and lost a lot of weight. We were all proud of her, but eventually she started taking it to far and now she is anorexic. She is extremely thin and is losing hair and she is really bony. Her mood is also a lot worse. She constantly yells at everyone, and has to measure food even when she's at someone else's house. How can I help her?