those who try to blame it on the mom run the risk of the mom during her weekend and the wedding emphasizing that stepgrandma must have not wanted the child at her wedding since she knows when the weekends fall. This could go on and on. It's probably the new husbands fault anyway. Does he not like children?
Sounds like to me that the stepdaughter is 100% part of the family. And the relationship between OP and mom's fiance is very good.