I think perhaps, your daughter is not looking at, and seeing you and your husband, as you are here and now, if you know what I mean.
It could be just that you're always there, and she hasn't stood back, and seen that ten years have gone past, you're in your 80's, your husband, her father is not so much another caregiver, as needing you to be caregiver, or it could be a bit of wilful blindness on her part. Or a combination of both!
You have so, so generously, given ten years of childcare gratis, to help them be able to save and get themselves financially stable. At this point, realistically, you are no longer in a position to be able to provide this for a baby, full time, plus the two older girls, plus your husband. She shouldn't be asking you, she shouldn't be assuming you are either willing or able to.
Now she and her husband need to do what so many people do when they choose to have another baby - figure out how to make their lifestyle and their budget work to provide for it - including childcare. This is their responsibility willingly assumed when they chose to have another baby, as parents and adults.