Well, for some signs I would look out for..

-Odd behavior (like an odd way of doing things for example)

-TALKING about suicide.. in anyway, EVEN if it's only joking (ex. "I wish I wouldn't have ever been born...")

-Giving away things, or sorting things.

-Wide range mood swings, one minute nice.. and one minute yelling at you over something so small.

-Cuts or bruises on arms

and.. just.. what you think, doesn't feel right about your sister. It's really cool to see a sibling posting like this asking how they can help. Most brothers or sisters I know wouldn't do this. Awesome job to you. :-)