My 8yo has ASD and has a lot of verbal and physical stims. My 7yo gets immensely frustrated by it on a constant basis. No matter how many times I've tried to explain that it's something he can't control, the frustration doesn't go away. Because 8yo has echolalia he will often copy you so my 7yo will sometimes even start crying saying 'hes copying me'
I've showed him videos of other kids stimming and I just can't come up with an idea to make any of this easier. It's everyday. All 8yo will do is flap his arms a bit or move in a certain way and 7yo will kick off.