Can a mother end up on the Sex Offender Registry if she was not the molester?
Yes she can. If a mother is found guilty of "failure to protect" she can be put on the sex offender registry along with the actual perpetrator. Does this actually happen? Yes.
Unfortunately, most of the sexual abuse survivors I have worked with told mom they were being molested and mom ignored them or called them a liar. This is so common I've come to expect it. However, new laws are being created to punish moms for "failure to protect". If the mom was told this was happening and failed to act on it she can be held accountable.
There is a Dr. Phil segment on YouTube which provides a poignant story of this exact situation. In this story, the mother's absolute refusal to take responsibility for her part in the abuse it classic - and sad.
Women can no longer wring their hands and cry and play victim where their children are concerned. I'm pleased to see the law stepping up and holding mothers accountable for the abominable practice of putting their men ahead of their children.