My 4yr DD old is *still* like this to a degree - i think it's a stage ( a long one in my case) but also a "first born" thing - they seem to think *everything* is theirs

a lot of DD1s clothes have been handed down to DD2 (2yrs old) and whenever she sees DD2 wearing them she says "but they're *my* clothes Mumma". i usually just reply that "yes you wore them first but because you're such a great big sister you've been very kind to now let your sister use them". she's pretty happy with that reply and it's forgotten about until the next *thing* that was hers comes up

eta:- when it comes to toys i always emphasise what a good sharer DD1 and how proud I am of her etc. she responds well to lots of positive praise and doesn't like it when i say how *disappointed* i am that she won't share with her sister