I wonder why and how siblings develop a competitive edge? Many many of them do.
Perhaps it goes right back to mama soothing the baby when older child also needs attention and touch?
Rivalry can spill over into the rest of life.
Some people I encounter only know they want sometihng when they hear that someone else wants it or is having it. Some people will happily have a small piece of cake until they realise someone else is having a larger one. Some people prefer to be the fastest swimmer in their lane. Some people want to read the book I just started rather than any of the other 20 lying around the room.
I have no idea how one combats it or engages with it with small children, except perhaps to encourage them to find happiness and fulfillment through their own actions rather than by comparison with the actions of others (this is a place where only children have a real advantage over siblings, I think - I rarely encounter onlies minding other people playing with their toys, or comparing helping sizes or whatever; I see it all the time among those with siblings).
- have toys and spaces which are really one person's, and if they say "leave" or "let go", their wishes are respected, in the same way that if the shopkeeper says "I'm closing now, please finish up, pay and go" you have to respect and adhere to that.
- having more parallel play going on, where one is baking cakes and the other is painting or something, so that the comparisons are irrelevant?