Discipline is one of the fastest ways a blended family can create resentment.
The biological parent may expect the new partner to "help parent."
The new partner may think living in the home gives them equal authority.
The child may see correction from the new adult as illegitimate.
Everyone can end up angry.
The solution is role clarity.
Relationship before authority
A child is more likely to receive guidance from an adult they trust.
That trust takes time.
Early in a blended relationship, the biological parent should usually carry primary discipline.