Boyfriends ex/baby mama

Im not being a bitter girlfriend and hating the ex-girlfriend, I’m genuinely happy that she wants to do this. I just don’t know if I can trust her

Should I let my daughter stay the night with my boyfriends ex-girlfriend and their kids?

Here’s a little back story, my boyfriend and I have been together 7.5 years. He has 2 girls and I have 1 (we have none together). His ex (they have 2 kids together) had wanted to make plans with us about my daughter staying the night at her apartment with her and the other 2. We all get along for the most part, except she JUST moved out of her mom and dads house and JUST picked up the responsibility of being a mom 7 1/2 years later. (The girls typically call me mom, and tell us about how they still stay with their grandparents a lot, she calls me when she does have them to discipline them because she can’t handle them). I don’t know what to do. Should I trust her? Or should I say that she can stay at the grandparents house with them but not with her? HELP! I don’t want to look like an ass!