I'd say leave it alone and just meet them with an open mind right now.
When I was just starting out in my relationship with my partner (I'm mono), it was very difficult for me to switch out of the typical "but why am I not enough for you?" mindset (in fact, I still have small bouts of it from time to time). However, I wanted to get to know my metamour better and did my best to not let my issues in my relationship with my partner impact my friendship with my metamour.
Yeah, everything impacts everything else, but it wasn't her issue to work through, it was mine, and I recognized that. Your metamour may feel the same way.
If not... if she does start going into a conversation you don't want to have, just let her know you don't feel comfortable talking about it. Hoping you have fun on your girls' night!