My niece who is now 18, her bio parents broke up when she was 1 year old. Her parents were young, her mom was in and out of her life starting at 2. Her dad found another woman ( who he married) and my niece started to something refer to this woman as Mom/Mommy all the Messy Bun Hair Mom One Badass Mama American Flag Vintage T Shirt referred to this girlfriend/wife by her first name to the child and corrected her gently ( you mean Sarah• when she said Mom) because her bio mom would get upset and my niece usually referred to Sarah as Sarah until about Kindergarten or 1st Grade, when she would refer to her as Mom because that was who her friend saw and such and my niece still calls her stepmom “Mom”. It is developmental and kind of peer pressure for a 6 year old. But I do think that Bio Dad should be plan “Daddy” and boyfriend should have the name added on. I guess I wouldn’t make a big deal about it because it is the love that your niece feels but honestly, this boyfriend and mom break up then I would talk to mom about bio dad being just Daddy and new boyfriend being Dad Name, but I also think it gets complicated when their are younger half siblings in the house too.