I have to say that you do sound very judgey about her and her family. Being married more than once is not a crime, nor is making bad choices. And being sensitive about ones looks or weight etc isnt either. I am sure her mum didnt set out to marry men like that, she quite probably had issues of her own that led her into this type of relationship.
And she has an untidy house, so do I and again, that isnt a crime! I wouldnt be happy if someone had tidied it up for me even if they had asked, never mind if they hadnt.
I agree that you do sound a bit over bearing, and you do seem to have an rather over inflated opinion of yourself and your importance. Alot of what you have written implies that you are somehow doing her a favour by allowing your son to love her and be with her.
If you choose not to write to her then fair enough, perhaps you could write to your son? If you want to see your GDDs again then you do need to grovel. And in the meantime, make a promise to yourself that whether you agree or not, you will follow their rules and not do what you think is right regardless of their feelings. They are a family and will put each other first, you must learn to respect that.
All that said, to use you for free childcare and then behave like this is wrong of them, but tbh it sounds like you are all as bad as each other, just in different ways!