In the pecking order of loyalties:
1) God
2) Family
3) Country
Palin had to do what is best for her family. A Downs Syndrome child needs a lot of TLC, preferably from Mom. Sarah is storing up riches in heaven.
I don't think we know Sarah's real reasons for stepping aside. There's no doubt in my mind that she was running an undeclared presidential campaign, right up until about two weeks before her announcement.
She was on the path to the White House, but something caused her to stop. I know what she said, but I don't buy it. If her family were opposed to her running, she would have let the country know much, much earlier that she wouldn't run.
Sarah has served in one public office or another for about twenty years. She's been a wife and mother since day one of her political career. That never stopped her before, and I highly doubt that it stopped her this time.