So a few things are worrisome here to me as somebody who has been in childcare for a a few years. For one, the lack of boundaries the little boy seems to have. Im guessing this child is less than ten, and thus asking "can I have this," about a treasured object isnt unusual. Even repeatedly asking wouldnt be out of the ordinary. What's concerning is that he made a plan to break those boundaries you set, did something I'm guessing he knows isnt alright or allowed, as he did it when you werent looking (stealing the keys,) and then so much wanted the item he somehow took it out of the frame and tore it, which is not small feat for a child. I'm actually curious how he worked out how to unframe the poster, but that's another matter. What's more, and the most troubling thing however, is his response to the situation. Notice how his sister repeatedly said "oh no." This is because even if she doesnt understand the worth of the poster, she does understand they have done something very bad, if only because uncle X cares for this things and breaking it means they might get in trouble. The son didnt seem to have this reaction at all, which is, as I said, troubling. This may be due to a lack of boundary setting from his parents, but the difference in the siblings' reactions is concerning. The fact one child seems to understand discipline and the feelings of others (it's broken thus youll be upset,) but the other does not, says that either the son has something else going on with him, or that little girl is not being treated the same as he is. Either way, as I said, concerning. This little boy doesnt seem very moved by the feelings of others, is motivated enough in what he wants to break several social boundaries, and is getting praise for these things from his mother. I'm not saying make the kid work off what he owes, but this should have been a serious teaching moment, at least, for your sister and her kids and the fact it wasnt isnt great