Lmfao I used to have a similar problem with my younger sister where she’d copy everything I do. It was to the point she’s never answer anything, she’d wait until I did and copy me even if she hated my answer. It grated on my nerves so much. And now my younger cousin does the same...

Tbh I feel like this is a universal older sibling experience lol

Honestly you have to learn to let the copying go, they’ll eventually grow out of it. Also if they’re doing it to be annoying show you’re not annoyed and they might stop faster.
Otherwise they might just be trying to get closer to you? Idk. There’s not much you can do if they’re not willing to listen (which sucks ik).