Lindsey, you don't sound childish at all.
If he was such a rat, the best revenge is to live well without him. Let people make their own decision on who to believe... I dare bet his friends know him for the lout he is. I doubt ANY would want him to date their little sister! And that speaks volumes.
I don't know why you should be obligated to even say hello to him. I think knowing what you know about him, if you think about it, you might even end up feeling a bit sorry for his new girl. He's most likely not any better to her than he was to you.
If you really want to get him sweating, be nice to her and ignore him completely.
I'm sure he's just hoping you're still crying in your pillow over him. Thank God you got out when you did! He's not worth worrying over.
HUGS