Find a new roommate. You might even get someone who will pay half the rent, instead of a third.

Find a new roommate and don't do her any more favors.

She'll be back. If he is this loser you say then it wont take long.

She'll be back right around the time she finds out he's stealing stuff from her to buy his drugs. Just get her house key first, then change the locks anyway. As for why to do both, the first is a half-showing of trust, to see if they'll give up the key. Regardless of whether they do or not, change the locks, because it's very easy to get keys made, and you don't know if she has spares. You don't want her coming over to your place to feed his addiction because it's "Better to steal your stuff so he's not stealing mine."

Oh, and also, when she comes crawling back, inform her that it'll be half the rent this time, not a third.

To everyone ripping into OP's sister and especially those telling OP to never help their sister again: Did it occur to you that the relationship is probably abusive? That the sister was very likely pressured into it by her boyfriend? That he's probably going to try to get her addicted to drugs so that she can't leave him? Did you even stop to think about that? Does it suck that she left OP with the bill? Hell yes, but it's way more of a concern that the BF has that kind of power over her when it sounds like she was trying to get away from him.

Does she not have a sister that can help her get out of the relationship? did the bf do something that op witnessed as abusive?(ie. physical violence, threats in the suster's life) can they not get the authorities involved?

It is a concern of mine, I have seen first hand the way he treats my other siblings with her around. But she thinks it is funny. Unfortunately because I have never seen or heard him do anything physically or mentally abusive to her the only leg I have to stand on is knowing he is a dirt bag.

Hey everyone thanks for the help. I am not worries about them stealing from me because she will be moving out of the state. I have decided that after she moves out she will not be allowed to move back in, this is now the second time she did this to me. The first time I chalked up to her being 18. I think I am just in awe from the fact that she doesn't see how much of a dirt bag he is.

Love biologically blinds you to your partner's (or partners') flaws, unfortunately. It is also possibly that he already has her addicted to something and that it warps her perception of him. I understand you decision to not let her move back in. If you are up to it, you can collect resources for women in need, especially for those in abusive relationships, and maybe even see if you can fill out some of the paperwork for the services, then keep them somewhere safe to give to her if she comes to ask you for help. It's also violence awareness... something, or at least close enough to it that you could email her information on identifying abusive relationships under the banner of spreading awareness of violence- especially if you put something like "pass this to x people/everyone you know, it could save a life" at the end. Also, if you do ever let her move back in for some reason, just curtain off a part of the living room and just put a folding bed/couch, table, and dresser. Standing screens would work too.