Advice on a family issue.

My brothers girlfriend is bipolar, schizophrenic, has manic depression, etc...She won't get a job, she won't finish school, she spends every day in their bedroom sleeping and watching tv. She refuses to get help. 
They are living together now and my brother does everything he can to help her but is neglecting him self. He forgets to eat, and is so skinny. I finally saw him for the first time in a month and he seems so emotionally drained and depressed. And has lost more weight since the last time I saw him.
I'm his big sister, I'm suposed to look out for him. We've always looked out for each other and helped each other when things got tough. 
It's so hard to see him going downhill and he doesn't realize it. I don't know what to do, or if I even should do something. 
I should add that they are 17 and 18
I know they are young but my brother is mature for his age and they think they have everything figured out for their lives.