So many useful comments here, thank you so much.
A few responses. First, to those thinking this is me wanting to be “better” than her etc, couldn’t be further from the truth. I’m trying to work out why I have this slight sense of being offended as it doesn’t make sense. I have openly admitted that I am frustrated that DN is being let down, but that’s a different issue.
@Takeyourownline To be clear, first SHE is the one insisting he has ADHD and getting very very upset if everyone does not accept that 100%. She has also said repeatedly she wants him diagnosed so she can get medication (which she seems to think will be a silver bullet). Also, I have NO issues with accommodations without diagnosis. Hell, without that, DS would have struggled even more and his high school in particular were AMAZING while we were seeking diagnosis. Although from your posts it’s clear you just think I’ve got issues with my SIL and that actually, I should accept that she’s making specific decisions she thinks are best for her child. That’s not happening really.
And no, medication is not the ideal response to ND. But it is often the ideal response for one specific type of ND – ADHD.
The issue, in part, as @SpidersAreShitheads (great name!) highlights later is that she’s demanding a lot, some of which is not practical and more importantly to me, is suggesting a lot of these behaviours are due to ADHD and that the answer is for the rest of us to just accept it/let it go, without being willing to put the work in herself.
@Dontlletmedownbruce – absolutely re the work!!!! And the hairdresser – aaah. Well done on getting him acclimatised. We never managed it and eventually found the best way was to let him go by himself so that he could sit there and grimly tolerate it without having to try pay attention to anyone else! He was the only 8 year old I know at the barbers alone!!!
@SallyD00lally – I said “dispensations and support” and that was wrong, you’re right. Support should be there no matter what. It’s the fact that she wants his behaviour excused or ignored (ie dispensations) that I find frustrating, particularly as irritating though it is for the rest of us, it’s actually HIM who suffers – he’s lonely and isolated, and because his parents are exhausted by it, his relationship with them isn’t great either.
In fact, I think that MIGHT be part of my offense now that it’s been pointed out – as @TheTwitcher11 mentioned, there was certainly very little accommodation made from either of our families for DS’ challenges, particularly from SIL (and my own sister). And the odd time he did something “different”, SIL, in particular, did not take it well. There’s one instance in particular I remember where she allowed him to do something that was always going to end in tears and when it did, she went on and on and on shouting at him until eventually I had to remove him from the house to protect him (think carry a tray with expensive glasses when his fine and gross motor skills were still very poor, then went ballistic because he broke one).
@iirbRosb – yes, I think you’re right. She wants all the support without the work. Perhaps also, in light of what I’ve just said above that I had not been consciously aware of, that offends me more than just slightly annoys me? Especially as I’ve done a lot of the work for DS, because I want to help him and I figured out that was the best way to do so, often while not getting support from our broader family.
@GertrudePerkinsPaperyThing – YES, I think that absolutely IS part of it. This idea that “ADHD” makes children “naughty”. And again, perhaps particularly because in the past, DS has been lumped into the “he’s naughty” category when actually he was just struggling with specific issues. It’s also about downplaying the issues and making me feel she thinks that if ADHD just makes children a bit “naughty” it’s no big deal?
@Thursdayschild2025 – ironically, while DS finds him extremely annoying at times, he handles him really really well. He’s a lot older but he has that hilarious ADHD calm-whenever-anyone-else-is-freaking out thing, and he uses it brilliantly with his cousin. He can stop him/tell him off, without losing it in a way that most of the rest of us find difficult. And DN absolutely adores him. It’s lovely.
On my way over to SIL’s house, years ago, I nearly crashed the car when I saw DN and DS about to cross the road. DS was at that age where he was allowed out, but carefully, but apparently SIL had sent him off with highly impulsive DN to the shop! As I watched, DN started to run into the road. My heart stopped. DS just casually stuck out an arm and grabbed him! 
Thanks again everyone. This has been really helpful and I really do feel like I’ve nailed it down a bit – it’s the accommodations she’s expecting without a) doing the work and b) after refusing to do the same for DS. Just understanding that makes me feel a lot lighter and like I can work through it because I do not like feeling that way.