Girl from my husband's past going to be at my sister's wedding... In her bridal party...

I'm in my sister's wedding as a bridesmaid. I've met all her bridesmaids and one of them is a girl from my husband's past. There's some history there because when my husband was 17 and on a class field trip he tried to jump off the balcony of the hotel because he was being abused in his foster home and his mom had just signed her rights away. She sat by him and talked him down and got a teacher after he came down. From what I know they never had an actual relationship. Just a drunk hook up at one point. I know it would be so shitty to drop out of my sisters wedding. Especially when she already paid for my dress and put down the deposit for all our makeup. And she's been with her girlfriend for 9 years and we were all wondering when they were finally going to get married. The girl has tried to be kind to me but idk what to say to her. She's also single. I think she might have kids but I know she's single.