I cited a study about this elsethread[1]. And "Lucy" was young (no older than 12, possibly as young as 7) when the rapes began, which correlates with a reluctance to disclose.
It is possible that the mother knew, but it is far from certain. The article didn't provide that context, because it chronicled detective work that led through a different chain of clues to crack the case. An obsession over maternal guilt has arisen here in the comments that was not present in the article.
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.
My mother worked in mental healthcare and she's always told me that "the sisterhood" doesn't exist.