You did the right thing. She may not have wanted to hear it but at least you did the right thing. Not many people can say that. And your sister is her own person so she decides if she still wants a person in her life who intentionally tried to hurt her. She may not have liked it but she chose to stop talking to you and in the end you did nothing wrong. She'll see that eventually (if she doesn't already) because if he tried his wifes sister then nothing and no one is off limits. You've warned her now what happens after that depends on how she takes the news and what she does with that piece of information.