I agree, you're expectations of her are unreasonable.
You are effectively saying that she knows you struggle with DS, so her and her family should continue having him round once or twice a year to give you respite and because he enjoys it.
You probably don't know everything she has going on in her life and if her family want this to carry on.
She may well feel sorry for you and DS, but if her DS and the rest of her family don't want this to carry on, what can she do? It puts her in a very difficult position. Her DP/DH might not want this.... is she to put your need for respite above her family? They may find it challenging to have your DS overnight. I know you have him everyday, but it's not their fault.
Of course she wouldn't want to hurt you by saying that, or that Bob doesn't want a sleepover etc.
You want her family to continue fussing over DS, but that kind of thing usually stops at a certain age. I'm sure he loves being the centre of attention when sleeping over there, because it's fun, but realistically, that can't carry on forever.
The siblings of Bob and his family who used to lavish him with attention have also grown and changed. These are all natural stages in human development
You're not actually considering any other reasons why things may have changed. Or because of the distress and hurt it's causing you, you're not in a position emotionally to deal with it by thinking of anyone else.
You need support and respite, but you can't depend on your friend (and her family) for that.
As you dont mention DS dad, your siblings or your parents, I assume they dont provide any support. Perhaps see if there is a support group for parents of children with disabilities in your area. If you can't find anything, contact your local authority.
I have some social worker friends who may know of something. If you PM and let me know the general area you live in, I'd be happy to ask them about support groups or they would know the specific department/service area that deals with this in CYP (children and young people) within the LA.