No she wouldn't IF that's what happened, however, Julie didn't mix with the relatives and they hardly going to tell Jeremy's girlfriend that they suspected him! She knew he was involved, because he told her.
I was giving Julie the benefit of the doubt, otherwise imo she behaved really badly. Maybe she didn't mix with the relatives but maybe she heard whispers from various sources which caused her to start to question and discuss her thoughts with friends. I would think that would be quite natural behavour at that age.
The alternative is that she knew he was guilty as she told the police and chose to party and sleep with him, even going as far as to deceive Colin into believing she was supportive of him. I understand all the arguments about her being under his spell, young, in love but for all that she seemed bereft of the natural boundaries which tell us where the line is drawn.