Well, you seduced him once, and betrayed your sister. If you're smart, you'll forget the whole thing, find a man of your own, and pray that your sister never finds out about what you've done. You really do deserve the torture of seeing them together and suffering your jealousy and yearning. You deserve to be miserable when he smiles at her. You deserve a lot worse for such a betrayal.
Sisters are blood, and men come and go.
Also, I suspect that he was doing a little getting even with you for rejecting him in the beginning by showing you what you missed. Get over it. I feel sorry for your sister, but not you. All the moaning and groaning and tears and wet whatever's won't change the fact that you blew it big time. First by rejecting him, and then by betraying your sister.