It's not a bad thing to encourage sharing, surely, it can only be a good thing. Mind, you have a good age gap that your youngest presents will probably not be as nearly as entertaining as her own. Of course she will want to play with her brothers too, it's natural for 4 year olds to be enquisitive of their siblings 'goodies'. I think it's a shame their grandmother thinks like that to be honest. We have a similar age gap with our children and it never really once crossed my mind to worry about how they will behave with christmas presents, and ds was so busy opening his own presents that, whilst he stopped to admire a new toy of his sisters, he was never inclined to open her presents and if he helped he was being just that, helpful.