I slept with the sister of my best friend
Why would you sleep with the sister of your best friend?
To get revenge of course, he often times does stupid things.
He is your best friend, how can you jeopardize your friendship like that with something so cruel? Your friendship will never be saved since you went to the point of no return. You don't deserve a friend like him and you need to think more clearly next time.
I don't care. I'll have no problem getting a new friend.
Well even if you don't care I think it's bad to sleep with a friend's sister to get back at him.