Nobody forgot it. There’s no evidence it ever happened.
You are right. There never is much evidence years after the fact of child sex abuse. I know this pretty well as do a lot of other adults. No one believes you. But her mom, grandmother, and brother remember all the constant petting of this child in front of everyone, ignoring the others, seeming to be “in love” with this one child, the long times they were alone up in the attic...
I believe it, I believe her. But no one in Hollywood does. They don’t really mind child sex abuse anyway.