Re: brother and his irritating girlfriend
*Since you're only concerned about your sister and her future, keep her close to you so she can rely on you at all times. Whatever she does (good or bad) she should be able to discuss it with you, because as i can see she can't go to her parents. But at the same time you have to give her, her space, no matter how much you tell her, she will only learn from her mistakes. You turned out to be okay so shes gonna be okay too. *
As far as the whole brother thing, believe me if your sis ever does something wrong, your brother will be the first to knock some sense in her. He sounds like a good guy, so give him some credit for that, because only a guy with true clean intentions would slowly intergrade his gf into his family. Which means he's putting his family on top of his list.