I would suspect she's choosing to be a stay at home mom because her mom made the same decision when Katie and her siblings were little. Just because a woman gives up a lucrative job to stay home and raise her kids doesn't mean she's brainwashed. Anyone can make money. Not anyone can love and raise your children the way you can.
Scientology is about brainwashing. If she were not a cult member, if she were an ordinary Christian or Jew, or even nonreligious, I wouldn't be surprised at her decision.