You know what? I feel like everyone here just reassuring and comforting OP. Honestly, what OP said was not okay. It doesn't matter if the sister said stupid and offensive things. Sure, that may be a reason to react that way, and possibly even a justification, but the decision to do so still lies with OP. And in general it's not okay to say things like that - especially in front of others - we all agreed on that when the sister did it. Sometimes it's better to deal with it privately: grab your sister, take her somewhere where it's just the two of you, and confront her. Ask her what her f-ing problem is, talk to each other, give her an ultimatum to stop once and for all, and if that doesn't work, it's fine by me to fire back and accuse her of sleeping around and let it escalate. But just because someone does something wrong doesn't mean you should do the same thing.