I keep remembering how an early poster said that this was thought to be a planned murder, at that time.
If that is the case, I don't think Sarah would have known about it. Daddy would have wanted to protect her from that knowledge, and not look like a mother-killing monster in her eyes.
But I think as time went by, when she maybe couldn't find any of Karen's usual shoes that were missing, when Karen didn't contact her, when Karen's birth family started to suspect ... I think the niggles seriously started with Sarah.
Maybe she turned to her friends then, went away, tried to escape the situation.
And maybe the police contacted her while she was away, spoke with her when she was removed from it all, maybe she opened up a bit then.