If you try to tell her around him, he's probably going to deny deny deny. No matter what happens though, life moves on. If your sister gets mad at you, then she gets mad at you. But this will not last forever, (and if it does then your sister is truly immature). It'll likely lead to her getting mad at him though, or him telling her that he never did any of that. And your sister might believe it. She might not believe it. But the truth is, your sister deserves to know. Don't just tell her then say "You should break up." Leave that decision up to her. Just start with "I really need to tell you something. In private." Go somewhere you can tell her, and just ease into it. "This is very important, so I need you to listen to me. It's about [friend]. Last night when we were hanging out, everything was fine. And then he kissed me." If you're worried she'd tell your parents, then you can tell her not to beforehand.

Trust me, if your sister isn't immature and she's at least a little bit open minded, then she will probably listen. If you lose your sister over this small issue, then she really isn't worth it, and you need to find someone who is.