I'm certainly not defending her - but if she was controlled, which IMO, it seems she was (and it fits, to me, his profile that he would marry a women who would be easily controlled and manipulated) then I think she KNEW what happened because she knew from talking to him, his internet searches, etc that it was a possibility and that this would be the modus operandi, so when he wasn't there, she already knew the plan because he had told her.
What I'm talking about is conditioning over a long period of time - trying to get her gradual acceptance of the 'inevitable' (Cooper's death.) Allowing her to research and discover and discuss what he was going to do, having her feel powerless to stop him. So, of course, she would worry about him being locked up for life / telling the police too much, etc because she was conditioned to be reliant on him, and she knew he murdered their son but wanted him to get away with it.
In her powerlessness, and assuming she was controlled by him, she probably didn't want to imagine her life without him. She may have been willing to accept her son dying in order to keep her marriage in tact, especially after a long period of conditioning and coming to accept the death of her son. This would be coupled with her religious beliefs and then branwashing her to believe that a negative deed actually created positive reactions for all parties (them as individuals, as a married couple, and for Cooper - in Heaven.)
I don't know - I have criticized her consistently in my postings and I have been rolling my eyes at her comments and behaviors. However, I'm just trying to look at it from a different perspective. Not that knowing and not preventing it makes her less complicit or anything, IF SHE IS INVOLVED (which I know, she has not been charged so all is MOO.)