A few thoughts on Jem... when Jem entered, the turf war between the Wolfpack and Aaron's Oh Dears was at its height. I always thought there were two crucial incidents that occurred during that time that turned her against Aaron.
The first incident was the stranded task walkout. Jem has said herself that she felt manipulated into walking out of the task by Aaron. The second incident was Jem's first nominations. There are a few videos of her talking by the pool late at night with Aaron, Faye and Harry. It's during this time that Faye realises she needs to start voting tactically to stay in, and what's really telling is that in the following nominations, Faye, Aaron and Harry all nominate Jay and Anton (no surprise there), and so does Jem.
In these two incidents, Jem acted alongside Aaron and Faye. Jem would have naturally been in the WP but, because she wanted to bond with her sister, she ended up on the other side, and hurt the people who were her more natural allies - Jay, Louise and Anton. I think it was her guilt at having done this that caused her backlash against Aaron; she could hardly blame her sister, so he bore the full brunt of her anger. I really think Jem was put into a difficult situation, and by chosing her sister over the people who would have naturally been her friends, led to the schism and disintegration of her relationship with Aaron, and the damage to her relationship with Faye.