The hardest part of this whole thing is this.... Your sister is in her "first blush" period. This means she's all twitter-paited with this new love of hers, and nothing anyone says to her is going to matter. She's got these chemicals going on inside her brain that's equivalent to crack. So talking to her is only going to alienate yourself from her. She's going to think "Why is everyone against me?".
Some advice is to get her to the side and tell her that if she ever wants you as a brother again, she will get his real name. I wrote off my own sister long ago for something she did. Another item that will go over well, is confront the guy. Yeah, its' ugly, but it's easier to get started if your doing some drinking. Get everyone else on your side first. I mean, who in the world won't give their real name? Get downright pissed off at him. I would.
and if she does get a license plate... PM me. (I have a friend).
No offense, but it sounds like she's a little young.... mentally at least.
....And if you really wanted to get the ball rolling, get some large photo's made of the Ex (or the Ex and her), and have them all over the house.
That sucks. I'm pissed off just reading about this scenario.