How to make siblings rivals instead of allies?

Reallysoawkward23

I’ve had siblings fight, etc and they still grow up to be ally siblings. What do I do to have siblings grow up to be rivals?

LaBlue0314

Do you have Get Together? If you do, you could use the Club system to help in this area. Make a club with only those you want to make at odds with each other. When they are in the club setting, they will do this all on their own, once you set up what you want them to do.

EgonVM

Hmm...
To make the relationship for siblings to show sibling rivalry once one of them becomes a young adult, you'll have to do a lot of mean interactions.
Try out this: Turn off aging and have two siblings constantly do mean interactions to each other for a week. Only mean interactions all the time. Then see if after aging one of them to young adult does show the rivalry or not.