I'm going to insult his sister in front of his life long friends
What makes you think that that is okay?
I really, really don't like him. This is just my way of showing him how I really feel.
Not liking him is fine, but why would you want to go so far out of the way as to embarrass him in front of his friends? That's way more energy than you should be willing to use on a person who you don't like and it's just a terrible thing to do. Just ignore the guy and move on if you don't like him that much.
He took my girlfriend when I was in a tour in the military. I've got no other choice.
I think that it is okay to be angry at him, just do not try to insult him in front of people. It is just going to embarrass the both of you.