The problem with your premise that any realistic mother would take her kid's side - there are PLENTY of examples IRL where a mother ignores her kid being molested by her new guy.
In
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, the grandmother says "“My daughter [Sophie’s mom] was brain-washed. Very naive and searching and just trying to find a man to love her," the grandmother said. "Their mother had never been loved by a man. Never shown any kind of care. So, when he started showing her love and care, she fell deep for it.”"
That sure sounds like Ann. She is a closet submissive. He's her master. She does anything he wants.