Of course it would end here. She was always going to be at the bar. While you might've imagined she'd have fifteen kids, or would've become dumpy, or begun a life of celibacy at a nunnery, odds are she's doing just fine. There was a reason you guys hooked up back in the day. She was smart, beautiful, and driven. None of that's changed. In fact, she's probably going more places now that she doesn't have you at her arm. Be civil. Be interested in what she's up to, but if she wants to do anything "for old time's sake," just leave right then.
If you do hook up, remember that both of you have houses teeming with relatives, so you'll have to find an empty parking lot or public bathroom. Now you're really back in high school. Awkwardness reigns.