A toxic sibling may "side with" your parent

In a well-adjusted family dynamic, there's usually no such thing as "taking sides." But when someone learns poor relationship patterns from a parent, they may try to earn that parent's affection by replicating those patterns and thus normalizing harmful behavior.

Toxic siblings often become a supporter of an equally toxic parent. They'll use similar critical language as the parent, and shame the targeted sibling regarding areas of life they might be feeling vulnerable about.