It's their family's business - this has to be the most horrible day ever for that poor girl. Seriously - this would be with her forever, if true.
It's not uncommon for grandparents to take credit for things like this. Would it have short-circuited her VP chance with right-wing repubs to have a 15-year old pregnant? You bet it would have. She did the right thing taking the blame. I seriously admire her for it.
HOWEVER -
She was selfish to accept VP nod. Knowing it would come out - and only a fool would believe the lie would stand up to the media - she put her kid in this tough spot. She could have declined mccain on personal matters, and left it at that. Good life as a Alaskan VP, everyone loves her, job security forever, etc.
instead, she let ambition get in the way of doing the right thing.