DD is 3,5 and a really sweet girl EXCEPT she just doesn't share with her brother who has just turned 1. She just takes his things, sneaks off with hers, snatches, says everything is hers and it makes playtime with the 2 of them really hard work. She is clearly still quite jealous of DS but does love him but just won't share - even attention. She'll shove him out the way if he comes in for cuddles, asks for books to be read if there is even a glimmer that i might be focussing on my son... it's a total pain in the bits 
So, is this an age thing (which i am happy to accept if she's likely to improve in a while), a jealousy thing or some sort of naughty behaviour i need to nip in the bud? i am thinking she'll grow out of it but it would be nice to get some reassurance!
sod the terrible 2's - this is way harder!!