Exactly why in all cases great and small the cops keep details to themselves that no outsider would know. I suspect that they can figure this one out pretty quick - either she knows things that no one else can know or as others have said it’s BS. OK she was only 7 and not the killer but around the killer but still.
If she knew at the time and withheld the information all along, then she is an accomplice. Claiming to have co-written notes and been at the scene of the crimes puts her above being just a "relation".