I have a DD, aged 3. At my MIL's birthday party recently, all the family was there, including DH's brother and his wife and their three kids.
BIL and SIL have never been good at disciplining their DC IMO. They will tell them to say sorry for hitting or to say thank you, the DC won't, and BIL and SIL will never follow through so the DC gets away with it. Perhaps not surprisingly, they have had problems at school because the oldest DC in particular doesn't often follow teacher's instructions. The DC are much better behaved with my MIL - their grandmother - as she sets them clear boundaries which she expects to be followed.
Anyway, at MIL's party, my DD and my youngest nephew - who's also 3 but eight months older than DD - both wanted to play with the same toy that belongs to my MIL. MIl sorted this out by saying that my nephew could have the first turn, then it would be my DD's go. DD accepted this and went off to play. But after 20 minutes, she asked her cousin politely if she could play with it now. He said no. I was present and so was his mum. She said nothing. I explained to the boy and my DD that he could play with it for five more minutes then it would be DD's go. When the time was up, he refused to give back the toy. Again, his mother stood watching saying nothing. I tried to get him to hand it over but he refused. DD, annoyed by this, took a figurine that went with the toy and went and sat on the other side of the room. My nephew was angry with her and demanded it back. My DD refused. I explained to him again that DD had let him had a go and now she was expecting her turn. We also sat/stood there in silence, me waiting for SIL to make her child hand over the toy, she didn't. Eventually my nephew got angry that my DD wouldn't give the figurine back and picked up the whole toy - a large bulky item - and threw it at my MIL's fireplace. Luckily it didn't hit anything to bad. My BIL heard this and his son screaming, came in, picked him up and carried him off. SIL followed in silence.
So I guess my question is, should this sort of incident happen again, would it be unreasonable for me to forcibly take the toy of my nephew (after first trying to make him give it back) even if my SIL is in the room? I feel as DD has struggled with sharing at times, it's important for me to show her that she will get her promised turn after she has let someone else go first with good grace. My SIL is such a sap, she doesn't like telling her DC off as she thinks that would make her mean Nevermind the fact that someone else always has to end up disciplining them.