Apron screams,, we have been in exactly same posiotion with my broother - she was always kicking him out, or making him sleep in the car. THe worse thing was he would come round and tell us what she would do , make us hate her, then go back to her.
In the end , because she knew we knew what she was like, and we always got on his case to leave her each time we saw him, he stopped coming round, phoning, and so we never had hardly any contact .
THe good news is 2 months ago he left her and has come back to spending time with all the family. HOwever, past experiences tells me that anythings possible, so Iwould never rule out them getting back together.
My advise, is just let him get on with it. Tell him your opionion , but only tell him once, dont keep harping on about it. Tell him you dont want a relationship with her, because u dont like her because she hurts him, and it hurts you see it .
And then like xales says, pick up the pieces, again. We mustve had my brother round at least a dozen times, swearing it was over, b4 goingback to her. Its just par for the course, its your sisterly duty