YTA 1,000,000 times over. If you’re transparent with your daughter about what happened to her biological mother, she will 100% understand that the woman she sees as her mother is not her biological mother. If you approach it correctly, she will know that her biological mother died in a horribly tragic way, but that she loved your daughter.

That doesn’t take away from the fact that she sees your new wife as her mum.

The fact that she’s “mum” and not “mom” leads me to believe you are not in the US, so idk how school works outside the US, but in the US kids start school at 5 years old. There’s lots of language like “tell your mom and dad xyz…” A 5 year old is not going to be like “well, my biological mom died, but now my dad has a new wife I can’t call my mom.” Kids simplify. The woman taking on the mother role is “mum.”

My bio dad isn’t dead, but he didn’t raise me and had nothing to do with me. My mom remarried after I was born, and that man is my dad. When I explain to people the situation, they go “oh, your stepdad.” No. He’s not my stepdad. He’s my dad.

It must be hard for you because it feels like your daughter is replacing her mother in your/her mother’s eyes, but kids don’t think like that. If she was older (like, teens) and you explained to her why it made you uncomfortable and she continued to call your second wife mum, it might be different. But she’s 5. She doesn’t know any better.