Your in trouble now, Oesdog - the Social Worker is in, and here it goes (just kidding 
): 
It sounds like your daughter is having a really hard time adjusting to motherhood. It's hard for everyone, but having it be an unplanned pregnancy makes it harder. I deal with that everyday at my job, and it can be overcome, but it will be a lot harder if you are doing so much of the "mothering" for her. And the less of it she does, the less competent and confident she will feel, and the worse the depression could get. If the doctor who told her that where here working with one of my girls, you better beleive that doctor would be recieving a call from me!
I know what it is to be depressed and a new mother at the same time. I lost my husband to cancer when my son was two months old, but even through the fog of grief I knew that if I was going to survive it with my little family of two intact, I had to do it by being the best mother I could be despite how I felt. I can't tell you how many times I politely declined people's offers to watch my son for me  for overnights (mostly from my in-laws). That's not to say I did it on my own - I had wonderful support from everyone around me, and my mother helped me out once or twice a week, but that was usually in my own house with me there. What kept me from sinking completely into myself was keeping my son front-and-center in my life. 
Obviously your daughter is not going through something quite that traumatic, but it is still a difficult time and the only way she and her boyfriend will adjust and come out of it is to do it together as a family. If I were you, I would set a very firm schedule for her of when you are willing to take care of your grandaughter, and for how long. You figure out what you can handle, and don't negotiate. Reasure her that she is capable of handling this change and will be a great mother, give her the help and advice she needs, but limit how much of it you are doing for her. You've paid your dues in this area, and in a way you still are with a disabled son. 
On a lighter note, Fried Green Tomatoes:
1 egg, beaten
1/2 cup milk
1/2 cup cornmeal
1/4 cup four
1 tsp salt
1/2 tsp pepper
4 gree tomatoes, cut into 1/3 inch slices
3-4 tblsp vegetable oil
Combine egg and milk; set aside. Combine cornmeal, flour, salt, and pepper. Dip tomatoes in egg mixture; dredge in cornmeal mixture. Cook in oil in skillet over medium heat until golden brown on each side.
Have a great afternoon, everyone. Enjoy those chickens!