From what I know, from experience as well, there are many ex wives who are friends with or take care of ex husbands, especially if they are ill. My aunt took care of her exhusband when he had cancer and was dying. In my own case, it has taken some time, but I am still friends with my ex husband. We don't hang out together but we have both agreed that we are there for one another if we ever need it. He came with me to look at a house I want to buy, I was there for him for a previous surgery and will do so again soon as he is having another surgery. We have kids and a grandson together so we are always going to be in each other's lives in some way or another. We also had 30 years between us before we divorced. I can't pretend we never got married or that he never existed and I don't want to do that.
I think that especially at your age, having that support is very important, both for you and for him, the fact your current husband is agreeable helps too. Your daughter, I'm guessing remembers the man your ex H used to be and is proje acting her own fears/hurt about her relationship with him on you. As an adult, she needs to understand that what you do is your business, you are not a child and you know what you are doing. You know him better than she does, in a different way than she does. It's a boundary between parent and child that needs to be made, even though I'm guessing she is in her 50s. My mother had a friendship with another man during her 60s, I understood what my parent's marriage was like, and as my mom was a cancer survivor...I figured life is way too short to be lived without being loved. Her friend was a part of our lives until he passed away.
My own parents are in their 70s, live in a companionate way, in separate bedrooms...for other reasons but they are like brother and sister. They do things together and take care of one another. It works for them. Your relationship with your exH and current H works for you. Your daughter needs to look after herself.