It is not the same situation as both children are untouched by trauma, however my DD (4) has had her cousin (4.5) staying this week.
Oh the tantrums! Younger one (DD) needs to be first, finish first, etc etc. I think it's a mixture of currently being an only child (though I wouldn't say spoilt) and the very real rivalry between them, both wanting to be better for some reason.
DD has just messed about while DN made a bracelet (they both had the same stuff to do this with) then howled when DN finished and she had barely started. She is usually a whizz at bracelets. Then DN cries and wants to go home...!
There have been similar episodes at the park. Always DD getting into a strop over something inconsequential (or even impossible).
Anyway, as someone looking seriously at adopting to complete our family (DH and I discussed this before marriage as I have always been keen to adopt) I now see that DD will need to be in a totally different age-band to any adopted sibling. I had heard this from SWs but have now seen it play out in reality, and as I said, without either child having other issues connected to experiences in care/ birth parents.
Am glad I have seen a bit of reality on this one!
(and any expertise from you experienced folk on how to deal with such flare-ups welcome!)