You have to move out. It's the only solution. He's not going anywhere and you'll be miserable. You could give her an ultimatum--him or me--but those tend to backfire.

The unfortunate fact of this situation is that your sister is a bigger problem than her boyfriend. Yeah, he's a bum, but she's a bum-enabler. She let him move in against your wishes, and now he's leeching not only off her, but off you too. It's like having a kid; she essentially added a dependent to your household, except there's no tax deduction for a loser boyfriend-in-law.

She also enables him by continuing to date him and letting him knock her up at will. The two of them together are a train wreck. If you stay, your relationship with her will deteriorate quickly, mark my words. Three is a crowd.

I admire you for not wanting to leave her in the lurch, but she broke the living arrangement when she let him move in. You have every right to bail. Who wants someone they hate living under their roof? Moving out is not only necessary for your sanity, but you'll be doing your sister a favor. Let her work her ass off to pay the rent while Slacker Boy does nothing but suck her dry, and see how long it is before she rips him off the teat and kicks him out the door.

You can give her a month or two to find a new roommate before you move out. If she decides during that time that she wants you to stay and him to leave, great. You got what you want and didn't have to give her an ultimatum. If she chooses him, then wish her luck--she's gonna need it--and be on your way.

Thanks for the question.