Wow, I've never seen them fight before! I've had the negative relationship to newborn babies, but I guess all of mine get over it. I didn't even know two kids could fight. Sibling rivalry indeed...I'll have to pay closer attention now.

I had this issue between a brother and a sister and it was more about their personalities. The girl was extremely shy and her brother was very outgoing. Her shyness caused her to reject some of his advances to play games or to chat and he was put off because she was such an introvert. The more he tried to engage his sister, the more she rejected him. They fought all time until he became a teen (he aged first). As a teen he was more patient with his little sister and she was more willing to allow him to engage her. By the time she aged to a teen and he was ready for college they had become BFF's.

Not surprise about siblings fights or their dislike for each other, a good example is when you first enter the Pleasant family in PleasantView their two twins girls Lilith and Angela always fight like cats and dogs.
Not only that their parents Daniel and Mary Sue have their share of marriage problems, they also have their fights while the two teen watch them do so.

When the young Sims go to a community lots a fight even break out there between some adults Sims, so sometime these younger Sims just do what they see from the adult Sims. It's like "Monkey See Monkey Do."
So >>Heimlichbourger >> Don't be surprise at what you may see in the Sims game, after all it is somewhat basic off real life actions, just enjoy the slap down.