A form of bulimia

My sister, who is also my best friend, is always complaining about how "fat" she is. She lost 20 pounds in about a month last year, and ever since then, she thinks she needs to lose more and more, and feels bad about herself for not being able to. So she has started taking laxatives. She takes three to five every single day! I don't know how to tell her that I am worried about her, because every time I bring it up, she turns it into a joke, "Oh yeah!! Sure...". I told her that I think it is almost a form of bulimia. Shitting everything out, instead of puking it out. Either way, she is purging.

Can anyone give me some advice on helping her, and talking to her in a way that she won't think I am condesending? She always compares herself to me, and tries on my clothes, and then gets depressed when they don't fit her. She is eight years older than I am and about half a foot taller than me... but she doesn't realize that she is skinner than I am comparitavely. I don't know what to do to help her! I don't think she is going to die anytime soon.. but I want her to live healthly.... Tweet