Would it be different if the mother left the father for this man?
Not in my opinion...Again, this is the sister’s day...
a) Is that a hypothetical or have you found it somewhere in the post?
b) Even if it is the case, primary responsibility for the adultery is the mother’s, not the stepfather’s. She’s the one who took vows. And if I’m a child assigning blame to my stepfather for breaking up my parents’ marriage, it’s borderline stealing for me to be taking his money for my education and wedding. Accepting his cash implies my acceptance of his relationship with my mother and of his stepping into a paternal role with me.