-I doubt your parents are separating simply because of this incident, it's probably just what triggered it. Perhaps things have been on the rocks for a while now and this just made it all to much to bear? Wa allahu a'lam.
-I don't think your sister is thinking straight (no offense of course) or clear enough to make a decision this big right now. She's probably just been swept off her feet and thinks if only she gets married it will be perfect, etc.
-If this dude did whatever it was he did with your sister, who's to say he wouldn't/hasn't done it before with someone else?
-You and your sister are different people and different genders, you think and feel differently. In general, from my experience, I'm pretty sure girls think waaaaaay more about marriage than guys do. We're more emotional whereas guys are more physical, so we'd be thinking all about the perfect wedding, the perfect guy, etc. So if she feels the need for a partner earlier than you do, it's completely normal, methinks.