Insisting upon good feelings between the children led to bad feelings. Acknowledging bad feelings between the children led to good feelings.
Thinking back to my own childhood and fighting with my brother: I just realized that while i was constantly reminded things like, "One day you'll only have each other," i don't think i was ever really invited to share about my negative feelings towards him. I was punished for being mean or fighting and told how i should feel, but it wasn't a thing to openly express jealousy, anger, or irritation.

