I think it IS good for families to continue to be there for each other even once the kids have grown up. I'm 32 and still occasionally need rescuing, and certainly need financial help even though I have a bachelors degree and a job. Adulthood is seeming to come later these days. I don't think its wrong for family to support each other-- the most magnificent structures always have multiple columns holding up the roof.
BUT
I think the support needs to go both ways. In my case, RAH and I repay some of my parents contribution by cooking all the holiday feasts and having supper at least once a week together. We are always available for family occasions, and we always be friendly to their other company. We make sure my dad, who has had some mini-strokes recently, is not completely lonely and eating alone every night when my super busy business trip mom is away for weeks. I make sure my kids spend lots of time with my parents as both of them just love love love kids and revel in reliving my own childhood in them (i don't know how many times I've heard them call my daughter by my name!). At this point the scales would still be really tipped toward my parents, as they do contribute more to the relationship. But I look forward with anticipation for the years to come when I will get to truly repay their care by being there for them when their final years come along bringing ill health and need.
One column can't hold up the roof, and if it feels like it is only you holding it together, I think its not wrong to ask for a little support for yourself too. In fact, asking for that is essential to your being able to continue to hold up your section, so its still totally helping them to help you. Maybe you can get that to happen, and can make your family into a beautiful six column cathedral, and maybe not, maybe they are not on board- you can't force them. But which ever way it goes, don't allow yourself to crumble into ruin!
First responders are supposed to make sure they are safe first as the number one priority-- the rule is to not add more casualties to the situation.
Although I wouldn't want to tell you to go NC or to not (your heart is the only one who can know what is right for you), I would offer this piece of advice about dealing with your adult daughter. Be honest, and also remain outside of the issue. I think its important to be honest about how much its wearing on you to watch the same crisis happen over and over. Hearing how you are recieving things might help her to see how repetitive the cycle is. Just make sure you are talking about yourself, how you feel, and that you are staying out of it. Its not helpful for friends or family to tell a person what to do... if you say 'dump him' and she doesn't, she may isolate from you out of guilt or resent you if she stays. I am so so so grateful for the healthy support I have received from my own parents when dealing with my AH/RAH, so glad that they never told me to do one thing or another.
I think its ok to be an ear and a shoulder if you aren't endangered by this and if you aren't actually taking charge and fixing things yourself. As PP said, take away the offer of a solution and you remove the cycle from the drama queen.
good luck and hugs!