You have the characters; now you need the storyline . Complex family relationships are not static—they are event-driven. The event does not have to be a murder; often, a birthday party is enough.
Families speak a private language of "pushed buttons" and shared shorthand. Writer's Digest Embrace Contradiction: real+incest+videos+busty+mom+and+pervert+son
One child can do no wrong; the other is blamed for all family failings. You have the characters; now you need the storyline
Do not use trauma as a backstory to excuse abuse. Use it to explain, not forgive. The audience should understand why the father is the way he is, but still hold him accountable for his cruelty. You have the characters