In real life, families don't break over the big fights (adultery, theft). They break over a misplaced vase or a forgotten birthday. The big fight is the straw; the decades of resentment are the camel. In your writing, ensure the trigger is small, but the reaction is massive.
A hidden truth (illegitimacy, crime, switched identity) is revealed, forcing a re-evaluation of all relationships. REAL SISTERS WITH PROVEN ID Incest - Google
She sacrificed everything—her career, her body, her sanity. Now, she wields her suffering like a weapon. The Martyr’s dramatic function is to induce guilt. Any attempt by the children to live independently is seen as a betrayal of her sacrifice. In real life, families don't break over the
Families do not speak in linear logic. They speak in code. In real life