I don't believe the question of Julie being an accessory has any basis. She obviously didn't believe Bamber right up until the moment the police picked her up at the Dartford Tunnel. My view is that she went along with his ramblings about his family for a quiet life but never in a million years thought he would ever go through with it. She was most probably in shock from the off and that is why she went to view the bodies and speak to Sheila.
There are 3 possibilities:
1) He didn't say enough things to put a reasonable person in her place on notice that he was seriously going to do it or
2) He did say enough that a reasonable person in her place would be on notice but she closed her eyes to it and convinced herself he was kidding because she didn't know what else to do or
3) She expected him to carry it out and felt she would profit as well from it and thus was happy about the idea.
There is no evidence that the third one is the case. There are disagreements over whether she should have known he was serious or not. Since we don't know exactly what Jeremy said only what she remembers he said we can't really judge that well. It is certainly troubling that he told her about the windows and planning to use a bike etc. While she said she felt he was kidding the greater the level of detail in the planning the more likely the person is actually serious. With that said people have provided details to others and yet lacked the guts/malice to
actually carry out such plans. So it is hard to say for sure what crosses the line where it objectively becomes where a reasonable person should believe the plan is going to be carried out. If he asked for her help then obviously that would do it.
Some people don't care they live with thugs without any qualms. A girl in my college dorm dated a guy who used to steal, beat people (not in fair fights he would do things like smack you in the back with a pipe- anything dirty) and she didn't care at all. She didn't encourage him to go steal for her but when he stole something she would take it and not care (receiving stolen property is a crime not that she cared). In modern society traditional morals are under attack so unfortunately this kind of Amaral behavior is more and more prevalent.
It appears that Julie had a conscience though it took a bit to get it to kick into full gear. Whether to rat out loved ones who do wrong is quite the dilemma. There are some parents who immediately will turn their kids, some who will do it in time and yet others will help their kids hide bodies. The faster one does the right thing the stronger their moral compass or maybe the less strong the bond they have to the person they turn in. Sometimes people aid criminals out of selfishness. They don't want to be without the company of the loved one or fear their name will go down the toilet for a relative being busted. That could have been her initial reaction that she wanted to not lose him and was being selfish in trying to protect him.