DD is 4.5. DSD is 8. DSD is spending Christmas with us this year and has been writing "lists" for about 3 months ! 
DD adores her older sister and basically wants to be copy her. DSD is, I think, quite "old" in some ways. Her list includes lots of things which I would have thought a bit too grown up for an 8 year but then perhaps I'm a bit clueless about 8 year olds ?? Anyway, that's a different thread I suppose.
My issue is that DD has basically asked for the same things as DSD. I know that she won't use/play with most of it because it is too old but if I don't get it, she'll end up following DSD around pleading with her to borrow it and that will just cause loads of bickering etc.
So, I suppose my question is, to what extent do you buy things for younger siblings just because older ones have them ??