As I said its some threads ive read in the past for example whereby a MIL bought grandchild its first ever Advent Calendar and the DIL was fuming as she wanted to buy the childs first Advent . Did it really matter? That sort of petty thing
The thing is, if you are aware that your DIL doesn't want you to buy her kid an advent calendar because she wants to buy the first one (and really, why the hell not, it's her baby) and you do it anyway, then you are, indeed, being a complete arse. If you are not aware and then the DIL goes mental at you, then it's quite likely you weren't listening or have previously overstepped boundaries in some fairly fundamental way. It really is the straw/camel thing a lot of the time. I don't see a lot of massively unreasonable DIL behaviour in the threads I read, tbh, though obviously there is some.
I'll tell you what I really fucking hate about my PILs, shall I? No? Oh, well, I'm going to do it anyway. When my DD was small, I often told PILs that I did not want them to feed her too much chocolate/sweet stuff when she went out with them for the day. The odd chocolate button, absolutely OK. Three packs of chocolate buttons a day, really not OK. They saw her once a week so it wasn't an irregular thing. DD used to tell me what they'd given her - often several large chocolate bars over the course of a few hours. I asked them for three or four years not to do it. They ignored me mostly and told DD not to tell me (she did anyway).
Fast forward a few years and their daughter has a little boy. He is now three. I decide to get him an Easter egg as DD has been given tons of giant inappropriate Easter eggs over the years by PIL and SIL so I assumed it was the done thing in their family. "Oh no" they say, "Little boy never eats chocolate, it's so bad for his teeth and health".
Fine. So your daughter's rules are OK but mine are not? In that case, fuck off the lot of you.