Im sleeping with my best friends brother?

You are older now so I don't think it'd bother his sister quite as much as it did before. I suspect that when you were going to her house to hang out with her or see your boyfriend she'd feel left out because you were probably kinda ignoring her and giving him the main attention. Which relationship do you treasure the most, with her or with him? Use that as a guide. But it does seem that if you have a really nice talk with her and explain how you now realize that you were ignoring her sometime before but will make sure you don't do that ever again that she might understand and forgive you now. You'll have to explain to your "new" boyfriend the same thing and when you're hanging out with her to make sure to hang out with her and when you're with him that she understands that too. If she still resents this new arrangement then I guess it'll be up to you if you want to take a risks of losing her as your best friend, or still not being able to get this in the open with dating him. However, why don't you at least talk with her and try to explain. It probably is bad for not letting her know so be sure to tell her how sorry you are and reassure her of how much her friendships means to you and that your going with her brother will never take away from that. Hopefully, you won't have a big bad break up with him at a later date that might affect your relationship with her.

well what's the reason she's so against it. it could be because she thinks your choosing him over her .in that case you need to reassure her you still treasure the friendship you guys have and make time for you guys to hang out without her brother.