This is a non-issue tbh, I'm honestly just interested to see how other parents handle these situations.
We were visiting in law's when SIL, her DH, and their 2 yo DC arrive. Their DC has a new toy and refuses to share with my 3yo DC. I don't say anything as it's not my place so I try to distract my DC, but, of course, DC is uninterested in my distractions. SIL's DH says he's not making his DC share as he doesn't want to deal with the tantrums (tbh, this is very much typical of him). Their DC never shares their toys, which is upsetting to my DC.
My DC has loads of toys at my in law's that belong to him (Christmas presents etc.) and they always (reluctantly) share them with their cousin. If DC has a tantrum over sharing, I handle it. However, WIBU to tell my SIL and her DH that, if they're not going to make their DC share with mine, then my DC won't be made to share with their DC and they'll just have to handle the tantrums?