My 6 year old GS sometimes doesn't want to play with his 8 year old sister. when he gets fed up he removes hmself to his own room.She likes organising him and complains loudly. The point is that as has been said children are just people. He needs some space of his own so does your GS.
Let him sort the toys. Some are his and little cousin is not allowed to play with them unless he gives her permission. Some are shared. Some are hers.
You could set up games to play altogether which might encourage him to share.
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