In my game, gender seems to be the primary factor affecting sibling rivalry. Children will readily accept siblings of the opposite sex, but are jealous of those of the same sex (perhaps they feel their inheritance is jeopardised). I've noticed this with several families, both Maxis-created and self-created. Then again, I'm not very awesome, so I could be completely wrong.

NooB sucking up to those more awesome than myself --> 
(actually I've been lurking for months, but never registered until today)

Cheers!