It sounds as though you may be mildly depressed, or at least too much is happening for you to cope with. Until things become clearer, it's best not to burn your boats. As your husband is working and will presumably have a reasonable pension when he retires, it sounds as though money will not be a big problem, although if you have no savings it might be worth starting to put something aside now. If nothing else you may be able to help your DIL with treats such as massages, beauty session. etc later on, or help the family to go on a break. Maybe you need to treat yoursef too as you,'ve been through a lot of stress lately.
It would be a pity to give up your job and then find you're not needed. Would it help if you could reduce your hours? Have you talked with your son and DIL (and maybe her parents) and told her that you will be there to do whatever is needed now and in the future, (she may be worried about how her husband will cope with the children if the worst happens)- tell her that you are ready to retire if needed, and that she needn't worry. It will help her cope with whatever happens to get all her worries about the future out in the open and talk them through. Ask her what would be the most useful thing you can do? It might just be chauffering her to appointments, or helping with housework, ironing etc. (Or paying for a home help to do it). No doubt things will change from week to week. You need to find a way to recharge your own batteries too.