With regards to Bristol:
I don't think that 18 is a great age to have a baby. I had my daughter at 18 and I've spent her whole life encouraging her to wait longer than I did. Like Bristol, I belonged to one of those hypocritical Christianist groups that didn't give a shit about women but enforced anti-reproductive justice principles. However, I made the best of my situation and I've done a better job than many of my friends who waited a decade longer to have kids.
That's why I don't think it's fair to really blame Bristol's behavior on her age. I was 18 and I made the decision that, regardless of whether her father and I would be together, I would always do my best to facilitate a relationship between the two of them. I also made the decision not to ever say anything negative about him in her presence. I firmly believe that it is unloving to do otherwise.
My mother, who I lived with at the time, gave me good guidance in these matters. She was a great role model because, when she and my dad divorced, she did the exact same thing. My mother would never say anything negative about my father because, as she said, he is a part of us and what she says about him might also make us feel as if she feels that way about us.
At the same time, my mother made it clear that my child was my responsibility and she wasn't going to coddle me and encourage behavior she didn't agree with by taking on of my maternal responsibilities. That's why I don't think that Sarah Palin has any responsibility to stay home and take care of Bristol's baby. Grandmothers these days aren't going to go along with sitting at home baking cookies and knitting sweaters any more. Nor should they!
With regards to S. Palin and her own baby:
I think that parents should feel free to make whatever arrangements are best for their child(ren). Sometimes that means having the father provide the majority of care, while the mother works outside of the home. While I have no desire to see S. Palin anywhere near a government I have to live under, it doesn't have anything to do with the fact that she isn't a stay-home mom. Her positions are more than enough to convince me.