I have DS who's 8, DD who's nearly 5 and DD who's 2. My approach to sharing is that you can have a few things that are solely yours - particularly sentimental or delicate things like a special present or collection of something - but that otherwise you should let your siblings share your things if you aren't using them yourself. The idea being that this kindness will then be repaid and everyone ends up with more things to play with.
DS, despite being the oldest, is actually the worst at sharing. He will squirrel things away and not use them himself rather than have his sister use them. Yet he is happy to share her things and the first to pipe up if he feels he's being hard done by. He received a playmobil set for Christmas that he played with on Christmas day and it's sat on a shelf in his bedroom since. DD is desperate to play with it but he just refuses. She's asked that I buy her the same one for her birthday but I don't see the point of having two when DS doesn't use his. DH thinks I'm wrong and that DS should be able to do what he likes with his property and that I should buy a second set. Aibu to think that DS should share it? DD is very careful and certainly wouldn't break it.