Step mom

So I’m a step mom to a 4 1/2 year old girl. My husband got custody of her, so I take care of her most of the time. I get her ready every day for school. I pick her up and feed her help her with homework. I love her as if she’s my own. She calls me “jazz-mommy” she hasn’t been late one day or absent since he got custody of her. When she was with her mom she has 7 absents and 5 tardies from mid August - October. Her mom hates me. I’ve never done anything to this girl for her to feel that way toward me. Well the reason I write this is because our little one has a Valentine’s Day dance on the 14th and two parents can only go. She mentioned it the other day to him that she wants to go so that means I’ll be out. Should I just let her go with them ? I’m sad about it all because I’ll be missing out. What do you moms think ?