I spent a couple of hours yesterday refereeing between my 2yo DS and 5yo DNephew, they were playing with DS' megablocks and DS is in the 'mine' phase.
Anyway, they both moved on from the megablocks with DN playing on his kindle and DS playing with some of DN's figurines. DS sat playing for 10 mins without DN noticing but as soon as he did he said to DM 'X is playing with my figurines, I don't want him too.' DM replied and said to DN 'take them from him then' which he then proceeded to do.
I was shocked to say the least. I brought it up and said it wasn't fair for DS to share his toys but DN didn't have to and that he should be encouraged to share.
My Mother's stance is that he doesn't have to share his toys as they are his belongings. 
A few hours later DS was playing with his megablocks again and DN tried to take some and play. I told him not to play with them as they were DS'. He cried of course and my DM hit the roof telling me I was being spiteful. I ignored her and once DN settled down, explained to him that not sharing was hurtful and would make people upset. He understood and said he knew that not sharing his toys would make DS upset. I said that if he really didn't want DS to play with some of his toys then he should keep them in DM's room where DS couldn't get at them. 2 mins later they were both playing with the megablocks.
However, DM is furious with me. How dare I upset her grandson like that.
WIBU to teach my DN about sharing?
FWIW - huge backstory but basically she treats them very differently and favours my DN in a very obvious way.