I think it was self-preservation. She was a pawn caught between the relatives' designs and Stan Jones' brief (essentially the same thing - take Jeremy out).
I think she would have known Jeremy was not capable of the murders. I think you are very kind to excuse Julie's behaviour by saying she was forced in one direction where I believe her actions were born out of pure jealousy
I might have thought more like you if she hadn't done the NOTW article
She didn't know Jeremy was responsible for the murders