Kinda get where you're coming from. I don't have DC with Sen but I am a Lp (ex useless and non-existent for several years now) with a physically disabled child and I have mh issues myself which are quite significant.
My father is too ill himself to do anything but mum does LOADS for my sister. Sis has 3 and mum has had them all overnight for at least one night every weekend for coming on 13 years now. My brother lives away but I live same distance as sis approx and she's babysat dd 2 maybe 3 times? Bails my sis out ALL the time (financially, practically - picking DC up from childcare, getting shopping, watching DC while sis has appointments or nights out, even does my sis' ironing for her every week!).
To hear my mum talk she sounds like bloody Peggy Mitchell bleating on about "faaaaamily" but that's only when she wants something. The reality for me and bro has been that our close friends have been there for us when family (esp parents) could have been and chose not to. With the slight exception of each other although geography/logistics were a factor there.
It does hurt. At least in your case they're as bad for bro as you.
I just basically accepted several years ago I was never going to get support so I don't expect or ask for it.
BUT I do think in your case, with a gc with Sen, having had the support of GP themselves THAT is pretty shameful of them.
I'd do a "I'm so disappointed" chat with them. Disappointed they don't care enough to support, you or their gc. Disappointed they expect your help with gran despite having an already MORE than full plate. Disappointed that they had the benefit of support from family yet Cba to pay that forward.
And I'd be very distant with them after that.
Lack of support goes both ways.
I think my mum is expecting I'll help look after her in her dotage, she's even mentioned in passing she expects bro to move back home or nearby. She's in for a shock. If sis is the one so deserving of all her time now, bro and I are of the opinion she can be the one to help out later. Goes both ways.
Have you been in contact with charities and support groups relating to your youngest child's condition? Do they have a sw? So that maybe you could get more support from those experienced with his condition and like pp have said slowly introduce to him and get him used to potential babysitters.
Must be so tough you should be very proud of all you've managed, inc keeping your family together. That is an achievement many don't accomplish.
I can understand - to a point - them being reluctant to mind dc3, but to not even give emotional support, just listen to you vent or tell you they're proud of you is disgustingly selfish.
"Also it yur dd is 16 surely she can babysit her DB for a couple of hours?That's what usually happens in families" - with NT younger children, rare in families with children with Sen, not least because some Sen conditions come with physical complications too which is a LOT of responsibility to put on a young-mid teen, I wouldn't do it. Plus the older children's lives are already affected by having a sibling with Sen they deserve a break too bit more stress.
"This thread is where it's finally clicked, if your parents were like that then as they obviously didn't want to do all the grunt work for their own kids, they're sure as hell not going to do it for their grandchildren, and any parent who thinks they're going to be different as grandparents is kidding themselves. Sadly." You're right, maybe these are just selfish people who probably shouldn't have had DC at all.
"They've honored their primary responsibility of bringing up their own children." Well as per above, not always and not without a lot of support themselves.
"it's his loss as he doesn't have any sort of relationship with my kids and my kids as a result have no real interest in him." This is true too, my mum barely has a relationship with dd and I think my bro's kids would struggle to recognise her.
My parents too, try throwing money at the problem with bro and I, we've long stopped accepting. It's not only them trying to solve the "problem" but there's usually strings attached too.
"My dad always makes sarcastic remarks about me sitting on my butt and doing nothing" wtf! I'd pull him up on that!
My dd is now 17 and as yours op sees her GP for who they are. And as a result has little to no interest in them. She speaks to her GP on her dads side weekly plus her gran that side keeps in touch on sm and they have little "in jokes" despite rarely being able to spend time together (they're a good bit older and frailer than my parents, live hundreds of miles away, ex doesn't facilitate visits).
Posters are focusing on 'well it's kinda understandable they're worried about minding a child with Sen" there's loads of other things they could do that would take the pressure off
Emotional support
Practical support in other ways (shopping, cooking, other housework, minding the older 2 when they were younger)
But they're choosing to do NOTHING at all! That's just outright selfish.
"Complete strangers helped more than they did" I've had that, when I had my second breakdown and dd was still quite young and we'd no support a lovely neighbour I barely knew brought round a Mac cheese and sweets for dd.
So, op, yes accept that's how they are (accepting does not mean condoning), and seek support elsewhere. And don't feel any more obliged to them, now or in the future than they feel to you. 