I think JM was in a dreadful situation, She heard JB openly talking about how much he hated his family and wanted them dead. She heard him talk about killing them and thought it was all talk....until it happened. JB then tells her it was a hitman and that if she goes to the police she will be seen as an accomplice as he had told her he was going to do it.
She can’t have known if she was coming or going with it all.
Maybe she feared she would be implicated in the murders because of JB’s previous comments.
I just don’t know...none of us do. All we can say is that her behaviour in protecting him initially was odd for reasons unknown (I suspect the above though) .
I’m as happy as I can be that JB is guilty as charged but I’ve read extensively about the case using original documents where they are available. Carol Ann Lee saw even more as Essex Police (I think) shared stuff with her that’s not in the public domain.
I also think JB has buried his guilt very deeply to the extent he now believes in his innocence. When something occurred a lifetime ago then it must be easy to distance yourself from it with time. Especially if you felt no guilt in the first place.