I can see that your DiL felt she was the only one left out but that gives her no right to send nasty messages and by now she may actually feel bad about her instant reaction.
My idea is to use something quite different to reset the christmas situation and avoid the firing line too, by putting someone else in that place who doesnt care or even need to know. Simply declare that the doctor has said that having checked say - your bloods or you might be aneamic- that it is not to do with any cancer but that you need to rest and do little and that you will be having another check up in January and see what if anything needs to be done then, but that you are doing too much and should be taking things easy and having early nights etc etc. The worry of your husbands condition is also affecting you and you should be relaxing as much as possible and therefore sorry you are not able to host christmas at your home this year.
Then you could perhaps suggest a meal or meeting somewhere convenient to you all, but where you could be taken by one of the family or go on the train or whatever and not be responsible for any of the arrangements. This way you can hope to spend pleasant time with your family, chatting and looking at the childrens gifts etc etc., but with a clear intention of sitting around and NOT being the organisers of any of it, apart from asking which one of them will be picking you up!! Then in the new year , you can carefully drip feed the new arrangments for your life, by mentioning that you are not going to some meeting or other , and how you no longer are the secretary for the WI or the garden club or whatever. This should at least make them see that you are needing to step back in general, that it is not just them , and they might begin to realize that as they are getting older, so are you, and their attitude of taking all that you do for them needs to alter and they need to step up to the mark!!
In the meantime you can also begin to mention other friends of your own age who are now no longer flying off to wherever to their families as it is too much for them these days. By implication you are telling them that this applies to you too. So as this is beginning to sink in, you could then in the new year , if christmas has gone reasonably well, suggest that you could possibly try and meet each separate child with their family for a meal at a restaurant or cafe which is convenient for you and them, or for example if you are a RHS member you could meet at a garden, which would allow you all to have a pleasant time walking round and then go to a pub or cafe close by for a lunch. There would be plenty of choice being after christmas, and you would get good service and have no problem getting a table and a choice of NON christmas food too. The one other suggestion I have is if you could afford it, look for a hotel that is about equidistant between the children and go and stay for a couple of days there and invite them to come and see you there. Whether you see them all on one day and perhaps organise a lunch somewhere nearby, or perhaps if you were near the coast you could organise a fish and chip meeting and going on the pier with the children, or inland perhaps there is a museum or national trust property that you have always wanted to see and you could combine that with them meeting you in some way. So you show that you still want to be involved as much as ever with the family , but that it has to be in a more sensible and reasonable fashion. This way you show no favouritism to one or other child and family, but you actually are really saying loud and clear by your actions that as my MinL used to say "Lockharts cafe is shut!" which meant she was no longer making the tea or whatever or she had finished serving and anything more could be got by other people!! So remember "Lockharts cafe is shut!" Pin the notice near your phone, and by the door and practise it for the future. Wishing you success in the new venture and you will find many good and helpful things on Gransnet and people can have ideas that you have never thought of. We do not have to agree, and you may think the ideas here are crazy but it is good to know that you are not on your own and many people have faced similar situations. Wishing you all the best and hope that the new year brings new ways of living.