Well, attraction is human nature. Even when you don't want it to be there. However, you can not act on the feelings.
I'm guessing you've tried to talk to him about it, but he keeps avoiding the subject (and you?) Maybe he felt weirded out by it, maybe he wished it hadn't happened, maybe he wishes for more. Maybe he and his sister/your friend talked about it and felt it would cause too much strife between the two of them?
But if he isn't communicating anything, moving on the best thing, even though you're disappointed. Because investing any more time/energy/emotions without any return would just get very frustrating.
I'd try to concentrate on other things and avoid him for awhile if that's what it's going to take to get him out of your mind.