I think that this issue is a non-starter, and like the general rule for American politicians: if you keep your family out of the political arena, they will be left alone by the media and punditocracy.
But some commentators apparently disagree (roundup), and think that conservative Christian voters will shun the McCain/Palin campaign.
Steven Den Beste, an atheist, seems to understand that Christian teaching has a different understanding of sin:
The Christians are not going to be too concerned about the fact that Bristol got pregnant without being married. They're not really going to be happy about it, mind, but they're not going to be too put out about it, either. What's important to them is what happens after that, and everyone in the Palin family is doing what the Christians think they should.
Rabblerousing attempts by lefties to try to inspire a Scarlet Letter response will rally support to the Palins.
That's also why Christians are not particularly impressed with lefty attempts to talk about McCain's divorce. The man went to war and went through years of hell, and when he came back he wasn't the man he had been before. The love was gone; the relationship was over. It's a common fate for soldiers, and a sad one.
He divorced his first wife. He then married another woman and has been happily married to her for 30 years. That's the part that matters. He couldn't save the first marriage; it was dead. But when he got married again, he was by all accounts a loyal and loving husband, and remains so to this day.
Christians know that people make mistakes, and that people sin. From their point of view, the only man who never sinned was Jesus. They don't cast out sinners, because they believe that everyone is a sinner. What they look for is people trying to do right as well as they can, to live as good a life as they can, and to try to make up for their mistakes and sins. And from their point of view both McCain and the Palin family are doing so. And as long as they continue doing so, Christian voters won't turn away from them.
Speaking from inside the camp, I think that Den Beste may be a trifle naive in assuming that Christians will act according Christian principles, but I suspect that American Christian -- and especially evangelical culture -- will respond this way to the Palin family.