My two kids get along fine — they are teens now and best friends. I took a class at their Montessori school about sibling rivalry and it worked! We DIDN’T try to make sure they both got the same amount of pancakes or always spent the same exact amount of money on them. We raised them according to what their real needs were and what their interests entailed. They both became grounded in who they are without feeling like they were missing out on something.
My only regret is that our family is not big enough! Children from large families grow up knowing that life isn’t about complete fairness and everything being even. Children who are raised to believe that (and yes, I knew someone who was like that with her two!), will not be equipped to handle working at a job where it may not be completely fair all the time (or not appear that way). As a parent, it is a balancing act of real life versus meeting needs. You just focus as much as possible on loving them as much as you can and meeting their needs. It’s worked for me! I’ve never felt like a referee.