The real villain of the Summer I turned pretty, is actually, Adam. Nobody else.
He cheated on Susannah when she was dealing with cancer.
He favored Conrad over Jeremiah, which gave Jeremiah an inferiority complex and insecurities, because he's trying to gain his dad's affection.
But at the same time, he's putting so much pressure on Conrad, and forces him to grow up before he's ready, forcing Conrad to become parentified.
He's not there to take care of Susannah when she's dealing with chemo and is dying, forcing her teenage sons to do it, when neither of them have the emotional maturity to do so. It should not have been up to either Conrad or Jeremiah to look after Susannah. No. That should have been Adam.
He's the reason the brothers have such a difficult relationship. It was strained before Belly.
Both brothers have issues because of him.
Jeremiah has an inferiority complex when it comes to Conrad. He's jealous of the attention Adam gives to Conrad, and project his issues with Adams onto Conrad. Belly has to walk on eggshells around Jeremiah, and gets shut down everytime she wants to talk about Conrad.
Now, Conrad isn't perfect. He's got his own problems. He tries to fix everything on his own, because he's been parentified. He thinks he's protecting Jeremiah by trying to fix everything on his own. And keep everything bottled up, because he doesn't want to be a burden. It blows of in his face, because he's not emotionally ready or mature enough to get parentified.
The fourth of July episode is actually an interesting way to showcase how Adam has damaged his sons. When they're told he's not coming, Jeremiah is disappointed, but Conrad's is happy. He's actively enjoying himself. Having a great time with everyone. He's smiling and opening up. Everyone has a great time. And then Adam shows up. Conrad goes into shutdown mode, while Jeremiah tries so hard to please his dad, but sees his dad Pay more attention to Conrad. That brings us to the fireworks scene. Jeremiah sees Belly and Conrad about to kiss and shoot the fireworks. He sees somebody else prefer Conrad over him.
Simply put. Conrad and Jeremiah would have a much better relationship than they do, if it wasn't for Adam. They would both be much better adjusted. They could both benefit from therapy