I appreciate your insight as a survivor, and am grateful that you are here and able to give input.
However, from a legal standpoint, what an individual reasonably should have known is not a subjective standard, but an objective one. Every individual will be held to the same standard, against the hypothetical "reasonable person".
Intent and lack of regard are two completely different things. In the former, the bio mom would have theoretically had to have intent that the rape of SP happen and/or continue. In the latter, the lack of regard could be just turning the other way; in other words, knowing, or reasonably should have had knowledge, but did nothing.
Like
@gitana1 mentioned in her example, if the child became withdrawn, child began doing poorly in school, child's activities suffered, child started pulling away from family, etc. etc. etc., the parent should have known. That is the level of responsibility a parent is supposed to have for their child. It should not be the law that the child should have had to directly told the parent, and the parent ignored and intended for it to continue, for it to constitute negligence. That's simply not what negligence is. If negligence constituted only what people subjectively knew, we would never be able to hold people culpable. That is why the reasonable person standard applies in our legal system.
As of now, we do not know enough about the mother to know one way or another. Perhaps there were no signs, this was a relatively new thing, and she is just as shocked. But if it comes to light that there were signs and they were ignored, she should be held just as responsible as if she had directly been told. That is just my professional opinion.