You can't bounce the sis and let the bro stay!!

Sounds like you're stuck with one bad roomie and one good roomie unless you can afford to live there without both.

Just a thought, how does the bro feel about his sis?? Is this fellow as perturbed with her behavior as you and your lady?? Reward him with one month free of his share of the bills if he can find her a new place.

If you do strike such an arrangement, don't mention it to anyone... he is, after all, still her brother...