You don't mention how old she is, if she is in school or working, or what her deal is beyond her pawningoff the baby. Doesn't really matter. that is appalling in itself. unless she is on drugs, in which case it's best that you have the baby. But I don't get the sense that your daughter is on drugs. Prob'ly just immature and irresponsible.
You're getting a big wallop of tough love, so I hope you are hanging tough. Maybe you already knew you had to quit being so darned supportive?
The thing is, you did your job, you raised your kids. They need to take it from there. So you really do have to do the tough love thing.
If she calls you for help in the advisory way ("Mom, what's the best way to clean the sippy cups when they get gooey?") you of course offer your wisdom, because that is your role as mother.
As grandmother, you get to visit with the kids, spoil them rotten, and hand them back to their parents. She should be his main provider of Everything. Not that you can't help, Some, but do less than you are now. WAAAAY less.
You don't have to be brutal, you can taper off. I get the feeling that you are quite warm-hearted and couldn't go cold turkey because you know how hard it will be for her. Tough crap for her. Raising kids IS hard, and she had better learn that NOW. You don' tplan to raise the child and his mother for the next 16 years do you?
Turn the tables, gently if you must. But get to the point where you call her when you want to see him/them vs providing care for the child on a regular basis.
She does need to figure it out for herself, so start making tracks in the other direction. more vacations, more hobbies, more time to yourself. you have earned that. Do not allow her to monopolize your time. You have nothing to feel guilty about. It's on her to honor her responsibilities.
Consider family counselling so she can learn from an objective observer that she is not acting like a mature adult and certainly not like the mother her son needs. And maybe she will continue counselling on her own as she finds out it isn't so easy to be an adult.