Siblings fight over the front seat and then defend each other to the whole school.

Two things that look contradictory are the same attachment. Daily friction with a sibling is about territory and attention, and it runs on an assumption that the relationship is permanent, which is exactly why the person who argued all morning will take your side in front of everybody by the afternoon. The arguing is not evidence against the bond. It is what safety sounds like up close.

If you fight constantly with a sibling, you do not have to fix that to be good to them. Just make sure the public half holds: back them in front of other people, every time. The front seat can stay contested.