I'd say they were equally guilty of the caravan burglary; but Jeremy killed his family and then tried to sell explicit pictures of his dead sister - a sister that he murdered. He took her children, her parents, her life, her reputation and finally, he tried to steal her dignity! Wadda guy!!
They were not equally guilty.
Jeremy worked there and knew how to steal the money.
He probably needed a look out and chose Julie as his only option.
I doubt that Julie pushed him into it. Jeremy has never claimed this. He testified he did it out of greed. And spent the money quickly. Although he later claimed he did it to show security flaws. Either way Julie was probably not told about her allocated role until minutes before.