One sibling can do no wrong (the "Golden Child"). The other can do no right (the "Scapegoat"). This dynamic, often orchestrated by a narcissistic parent, breeds decades of resentment.
Looking at the landscape of television and literature, certain narrative engines have proven timeless.
Think Mo’Nique in Precious or Logan Roy’s late wife in Succession. This character holds the moral (or immoral) compass. In complex family relationships, the matriarch is often the polestar—either the glue holding the wreckage together or the wrecking ball herself. She uses guilt as currency and memory as a weapon. A great family drama storyline almost always hinges on whether the matriarch will heal the family or burn it down to save her legacy.