Girl who's a friend of boyfriend's family?

I've been dating my boyfriend for a year and a half now. We have a great strong relationship. This Friday his mom's friend is visiting with her daughter who is around our age (she's maybe 16 or 17). My boyfriend's mom has been friends with this woman since she was younger and they frequently hung out so my boyfriend pretty much grew up with his mom's friend and her daughter.

At one point a few years ago my boyfriend and this girl liked each other but since they'd known each other for so long and his mom described it as "since they're basically almost like cousins (even though not literally)" and grew up with each other they decided it was awkward and wouldn't work. My boyfriend doesn't see her too often though, the last time he saw her was last year.

My boyfriend's mother kept telling me ever since I started dating her son how I'd like this girl because we'd have a lot in common. Friday I'm potentially meeting her and something about it makes me uncomfortable. Just knowing my boyfriend liked her and that she liked him at one time makes me wonder what'd happen when she's around him when I'm not there or how he'd act around her or how she'd act around him. I'd fear flirting or something generally awkward... maybe I'm worrying too much.

advice on how to not feel uncomfortable about this all?