the thing is you say she LET him do drugs. she didnt force him to do drugs. it was his choice so you shouldnt really be blaming his sister. if your boyfriend said he will stop for you, you need to trust him. have you told him how you feel about him taking drugs? is sister might not even like him taking drugs but she cant really stop him if he wants to do it. you need to ask yourself if he would do it if he was with his sister or not. the problem is with your bf not his sister.

Get out of it, because when his sister is not supplying him with the drugs, he will come to you for money, and if you refuse, day after day, you will find your stuff missing....... for example, i have an aunt in the same position and she got out of it. She had a computer, she had a tv, she had everything in her place, until she could not take it anymore. Soon, it may happen to you as well. I am not saying it will happen, just that it might, so protect yourself and your home.... Hope you take this seriously, he may become violent when you refuse to give him money and so on... Remember, i am speaking from experience. Hope this helps...

omg, sister, no i know EXACTLY HOW YOU FEEL! my boyfriend hangs out with an occassionally sketchy crowd... they all smoke weed/drink. in fact, his two best friends, one in town, and the other out of town, are real addicts. they are fine ppl, but they smoke weed, drink, and do other, heavier drugs that I don't approve of [speed, X, cocaine, etc.] and i trust my boyfriend around them [even though i know that he has done some of those drugs before, and drank and smoked weed. even though you know that he will stop for you, you still worry about the peer pressure she may impose, or the fact that, because those sorts of things will be so readily available to him when he is with her, he will more likely to do those things, no? i feel the exact same way. the best thing to do to put yourself at ease, is to explain to him that it isn't that you don't like his sister, just that you question her lifestyle choices, and you would prefer if he doesn't do the types of things she does [drugs, etc.]. let him know how much he means to you, and just how important it is that nothing bad happen to him. once he knows that you are just looking out for him, you both will be put at ease... you especially. :]]]]

You say you trust him, but it really sounds like you don't. You can't ask him to stop hanging out with his sister, family is family and you're just the girlfriend. So, you either need to give him the benefit of doubt that he isn't doing drugs and alcohol or you need to get out of the relationship.