sounds to me like you've got no lasting hard feelings towards him for what he's done, so why shouldn't you remain friends?

what has happened with your sister is really none of your business. unless she has asked you to stop being mates, then you might consider it.

anyone trying to be hard with all this talk of hurting him to "protect" their sister is not considering her at all.

family will always be there, but it's hard enough to have good mates and keep them through life without things like this getting in the way. over time, the family will lose their animosity towards him. it won't take long for the next boyfriend to come along and it'll all be forgotten.

try and keep him as a mate is my advice.