Honestly if you hadn't noticed anything at all I'd assume she was fucking with both of you

My first thought is that the roommate wants op out and possibly can't just kick him out legally, so he had his sister call with the crazy story so op would move out on his own.

You're definitely a very manipulative person if that's the first thought that you came up with, but it's certainly possible. Unlikely though

Either he is good, or she is lying.

That could very well be true, but how do you know the sister isn't crazy? How do you know she is credible? If this individual has been your roommate for a fair amount of time and you've never had any problems with them prior to this conversation with their sibling; I would be hesitant to make any rash decisions yet. Good luck OP!

What if his "sister" is really his mother and without you paying half the rent, he'll have to move back in with her.

Plot twist: And, ... his mother died 7 years ago! Dunh-dunh-dunh!

I was with you up until the plot twist. For that reason I cannot give a thumbs up. Lol.

Or it was all a ploy to get OP out of the apartment.

What makes you believe her? lol

This feels like such a weird FML. Either you already felt like your roommate was crazy, and sought confirmation from his sister - which seems a little odd. Or you've been perfectly fine living there with your roommate and his sister randomly spent an hour telling you that your roommate is crazy, but how would you know she's telling the truth if living there has been fine? It just seems like it would be really easy for the sister be pulling a prank on her brother, which would make you the gullible party. OR, you've already been struggling with your roommate, which makes the sister unnecessary.
I should also note that a person can be afflicted with any number of mental illnesses and still be a perfectly fine roommate, making this an overreaction. I do admit there are some mental illnesses that would be less tolerable for a roommate, though, especially if left ignored or untreated by their bearer.

I agree with all you've said. But I imagine when the sister says "crazy" she doesn't mean mental illness, but he's just a thief, liar, and jerk. I think if he actually had a mental illness the sister would probably be more sympathetic. People tend to use the term "crazy" for anyone that they have issues with, who lies or steals all the time, or who isn't consistent with things, making people feel their reactions are unwarranted and come out of the blue. I agree that people call people with mental illnesses crazy too, but in this case with the way the sister worded things ("thief" and "liar") makes me think it's more an exaggeration.

He actually had schizophrenia and bipolar disorder when he didn't tell me when I moved in.

A thief and a liar does not "crazy" make.

True, but they don't make good roommates either.

What a horrible sister... maybe your room mate does have some metal issues... but maybe he has gotten the help he needed, and is doing fine now. It's sad that he doesn't have support from his family.

Watch your roommates actions and see if he goes back on his word a lot. Google narcissistic personality disorder and antisocial personality disorder. Both of these are thieves and pathological liars. In the other hand, how well do you know the sister? Alternatively, yawn in front of your roommate and see if he yawns too. If he doesn't- you've got a problem.