Paddyann, I am in agreement with you to a large extent but I think it is about getting the right balance. It seems to me that vwaves is having trouble fitting in everything she has to do and make friends so Warrington feels like home. My suggestion would be to define explicit times when you can help out with the children and do no more until you have your life balance to your liking. As for the husband, he has chosen to take on kids, having them at weekends is what parents do, that is not down to you. He might not have realised he was taking on a wife with CFS too but they need to sort out a way for them to have a bit of a break without you. I would suggest they look for a children's club or something at the weekend that they can take the children to so they can let off steam. For the little boy, in particular, he might enjoy cubs, football or hockey club or many other sports things that would burn off some steam. Even better, if his father could get involved so he could build up that bond which would make his son more responsive.
I have a daughter with EDS and CF is a major part of that, along with digestive issues, eye problems, etc, so I understand how obliged you feel when they can't cope. I more than empathise with the negativity and a reluctance to do what the professionals ask. However, we got Adult Social Services involved and she now gets a direct payment for help when her husband can't step in. Does your daughter get PIP? If she struggles and needs help this is another way of having the money to pay someone to help. My DD uses some of hers for her 6 year old to go to cake club once a week which is straight after school.
If you can get some regular time to yourself, you too might be able to find a club or group where you can start to make friends. You will need to make it an absolute time where you are not available to anyone else. The other thing which may be available for you is "Time to Talk" or even a "carers" group. The former would give you a chance to talk to someone about how to sort out your feelings about your relationships, life balance, mild depression, etc.
I think if you walk away without trying to make it work (and 18 months really isn't that long even if it feels it), you will feel guilty so you still won't be truly happy. The thing that shines through is how much that relationship with a difficult little boy who had such a terrible start in life, means to the both of you. It sounds like you are his salvation and although it isn't necessarily a responsibility you wanted, it is an honour too. I would suggest that his parents went to parenting classes (they'll come to the house if your dd is too fatigued to got them) so they can learn how to give the little chap the support he needs.