Thanks everyone. Everything's calmer now.
I sent her a reply saying that I respected her feelings and that it was my mistake not to have stuck to the original plan of meeting her at the hospital and I had offered to take her out for a meal to avoid such a situation, that emotions were running high and that I could not cope with any more drama.
I also said that they were all my daughters, that I loved them all, but that my priority is taking care of Dad right now, that I don't have the emotional energy for anything else. I said that I can't fix the relationship with her sisters, only the three of them can do it, and only if they want to; that DH had got everyone together because he didn't want the next time they met to be at his funeral, but ultimately he couldn't fix it either; that the past is the past and we can only move forward, even if moving forward doesn't seem like moving forward; that situations like this can bring families together or they can drive them further apart; that his illness and death will shine a light on what was, what is, and what will be, but that we will all be changed by it.
Today I have received a much more measured reply, saying that of course Dad is the priority, and that she did not mean to go on about the past, she was just effectively 'getting it off her chest'. She wants to bring the DGDs to see him, when he is out of hospital. All the DDs have agreed that DH is the priority and that they will be civil to each other for his sake, but I think that after the funeral, it is unlikely that DD1 will see her sisters again for a very long time.
Mary and Bibbity, you are both right, they are adults now, and my role is no longer that of the 'fixer'.