This is obviously bothering you a lot, and that's probably as it should be, but if you are so close to your sister, you won't be able to keep it from her for long. She'll know something is wrong and what's she's thinking is wrong may be worse than what actually happened. Yes, he was wrong and he's a puke and what he did is disgusting, but I feel like your sister should be told. She deserves to know and then let the chips fall where they may.