I think you can bring it up to your friend. Presumably she knows you slept with her brother, right? Maybe say,"yeah, just wanted to clear the air about that if there's any weirdness. Oh, also, I never heard anything from him. Do you think he feels awkward about what happened?"
One of two scenarios is currently playing out:
1. He's avoiding you because he feels awkward. You're his sister's best friend. Maybe he thinks your sister will be weird about him sleeping with her friends, or that dating you means his sister will know everything about his love life.
2. He's attracted to you enough to sleep with you, but he doesn't want to date. Maybe he thinks you do, and now he doesn't know what to do. So he's taking the chicken route and avoiding you. Was alcohol involved? That will increase the awkwardness.
Talk to your friend about it. And since he's someone you've known for so long, there's no reason why you can't say something to him. Say you think you should talk about what happened and want to clear the air and make sure there isn't any weirdness between you moving forward. While he may not be a player, he's showing a lack of character by avoiding you. So, overall, he may not be someone you want to date. But since he's presumably someone you may see again, it will be easier for you if you talk to him about what happened and minimize any future weirdness.
Disagreeing here. I would keep your BFF and his sister out of this! You want answers - this is between you and him - she should not be put in the middle. It will be tough enough for her to be neutral and not pick a side...
sluttty.
She's slutty because she had sex with a guy? please...
how do you help someone by calling them slutty i wonder? sheesh. i don't think its slutty. this is a tricky one though, i think nick handled it well. if it were me in this situation i would try and figure it out with the guy before discussing it with the BFF. UNLESS the BFF brought it up first, then it would be all cards on the table, thats just me tho.
I agree and disagree with Nick. It's unclear from the question whether the OP has actually made any move to contact the guy herself to get answers rather than leaving it up to him to call her, so if she hasn't done that yet she should do that before going to the bff. If that fails...well, I personally wouldn't mind if my bff told me that she'd slept with my brother and needed some answers, but a lot of women would. In the OP's situation I'd consider carefully whether to put my bff in the middle, and if she didn't already know that I'd slept with her brother, whether to tell her at all.