Thinking of this scene from season 2
So I keep thinking of this scene in season 2:01. Jere says he is pissed at Conrad, and then immediately says "that's why I think we should tell Conrad about us. I don't want any more secrets." To me, this makes me feel that part of Jere's motivation to be with Belly is to get back at Conrad. We saw it with the firework, too. I mean, I know there are different reads on this, but it just seems a bit sus.
There is a scene in Book 3, where Belly catches Jere's eyes flickering over to Conrad after he kisses her in front of everyone. To me that is when Belly realizes that one part of why Jere wants her is to "win" over his brother. It is unhealthy, it is cruel to Conrad and it doesn't bode well for their relationship.