It’s not just "kids being kids." One child discovers a secret about the other’s biological parent that could destroy the new marriage. They use this information as leverage, not out of malice, but because they believe that breaking the new family apart is the only way to "save" their original parent’s memory.
A sibling who defended you in Act I may betray you in Act II when their own interest (spouse, child, money) is threatened. Genie Morman Incest Family 272
Boundaries do not exist in this dynamic. Parents live through their children, and secrets are treated as currency. The drama arises when one member tries to break free and establish individuality. Core Storyline Elements in Family Dramas It’s not just "kids being kids
The one who ignores their own needs to keep the "house" from burning down. Boundaries do not exist in this dynamic