This is something DH and I are struggling with a lot in the context of family member and her child (someone we see regularly at extended family events and she lives close by so there's a certain sibling/cousin type interaction with regular contact).
The child has always seemed to us to be very hyperactive and sensory seeking. But it also just felt like attention seeking as his parents' relationship was truly toxic and often the only time they paid attention to him was when things were really bad. Lots of ineffectual "don't do that" type stuff but largely ignored until things got broken or had reached epic proportions..
But he's 9 now and things are getting worse. Mum is single now (good thing because dad was a complete wanker. Needless to say, he's an absent dad now). She thinks he has ADHD and is seeking support for that - which is good.
But there's still little proactive parenting and we can't work out howm uch of his behaviour is due to potential ND and therefore deserves more understandign and patience, and how much is just him being spoiled. Eg he has a meltdown and his mother will expect the rest of us to accomodate those such as screaming and shouting, stealing our DC's stuff etc because "he didn't mean it" or "he's overwhelmed".
it's a difficlt line. Currently we mostly navigate it by trying to limit somewhat the time we spend with them, and also by pre-empting certain things - eg we have made it clear that he's not allowed to go into or DC's rooms and mess with their stuff and have told his mother that. She's clearly not wild about it, but has accepted this boundary. But our DC are still irritated when he's calling them names or grabbing food or whatever and they're not allowed to do that. And I'm not sure what to tell them.