Blended families can often facilitate a level of competition between the adults. The biological parent who the kids don't live with wants to prove that their more important than the stepparent the kids do live with or vice versa. Sometimes, a stepparent is just objectively more of a parent to a child than their biological parent, and that can foster animosity fueled by feelings of inadequacy on both sides. How should a stepdad who has been a father figure to a kid his whole life feel when his biological father shows up after 9 years of absence and takes his place in the eyes of the kid and his mom? It's one thing to prevent a child from having a relationship with their biological parent because your adult feelings are hurt, but if they're doing none of the work and getting all the credit, it's definitely a situation worth evaluating.