I think this is fairly normal-if irritating.
One thing I remember saying to dd1 was that it was absolutely fine her wanting to have a specific thing (like the purple plate) but if she had a quiet word with me rather than making a fuss about having the purple plate then her younger siblings were less likely to also want it.
It took a long time before she did actually take that in enough to not do that 🤣
Then she got rather too good for a while at calling out "I must have the blue plate" and letting her siblings fight over that one while she quietly picked the one she really wanted. 🤣
What I will say though is try and give her some interests that he doesn't do, or at any rate if he does do, he does with a different group. So if she wants to learn the piano, he learns the flute type of thing.
Dm used to push my younger brother to do whatever I did (I didn't tend to do what my older sibling did), and I really found it difficult. For a start off it often meant my things were taken from me and given to him when he needed them and I was then using something that was too big for me (he was only slightly smaller than me, although several years younger) and also I felt I could never match up to him. If I did things better-well, I was older and it was expected so not worth anyone saying things. If he did thing well, then wasn't it amazing, he was challenging me and everyone should be really impressed.
It mucked our relationship up for years and I felt I was never good at anything.
Also make sure that she can have her own space to do older things without him. If she's friends round let them play without him, if she's doing something that is her interest, he doesn't have to join in etc.
When she is doing her own thing, it's a perfect time for him to develop his own thoughts. Let him choose something and do it.