[rectang]

No one wants to admit that their child was raped, even if they are open to the possibility — so your assertion reduces down to "there often is a realm where they should have known".

Because the article doesn't give detail, we don't know. The mother could have forcefully spurned explicit disclosures from her daughter. She could even have participated in the abuse.

But there's also a possibility that since the perp was clever enough to hide identifying details while publishing CSAM online that he was clever enough to hide abuse from those close by.

I would condemn participation but forgive ignorance. Other commenters here will never forgive the mother no matter what.