All the women on my husband's side of the family have a terrible time saying no to anything. They are almost always overstretched, stressed out, and have chronic health problems of their own directly related to stress. I was not raised that way. If someone asks me something, I assume it is because they want my honest answer. If they don't like it, too bad. I am not mean about it and I try to be considerate of them, but like I tell my kids, "If you ask someone a yes or no question, you have to be prepared for and willing to accept either answer. Otherwise, don't ask." My sisters-in-law all tell me they wish they could be more like me in that regard. My point is, I think you are well within your rights to sit them down and tell them you really wish you could do it and if you were well you would jump at the chance, but right now your health is not good and you have to have the ability to have a lot of rest and peace and quiet. No way your house will be that way with kids there. No need to feel guilty because you are telling the truth. Ask them if there is any other way you can help them or give them some ideas of ways you can somehow help out a little, if that's possible. I agree, you have to take care of yourself first. They are adults and will come up with another solution. But you should not feel guilty.