Hi Booya, thanks for playing our game!
You're right that Crystal and Brad getting back together is a bit sudden at the end. The reasoning is that Daniel's sense of pride about working with his uncle and father and fighting for the people is so strong. That's why you see him in the end marching along with the combined Remnant/New Dawn forces, presumably keeping peace over the land. This always meant more to him than Crystal, even though he liked her.
As for Brad, if you take time to speak with him in the final Sanctuary scene, you'll read that he's regretting what he did. He's realized how foolish it was to fight with the Remnant. That's why, in the end, he decides to leave the military life behind and get back with Crystal.
Brad and Crystal's back story was never shown. In the game you only see Brad as an obsessed Remnant member, being rude to Crystal. I can understand why you'd rather have Crystal end up with Daniel! But Crystal never forgave Daniel for leaving her behind after seeming to help her, only to "use" her to try and get her to join the New Dawn. She and Brad have a lot more history together, and he left out of a sense of duty and not because he wanted to break up with her, so it's natural that they would end up back together after the trouble was over. Daniel was just a rebound
![Wink ;)](./images/smilies/icon_e_wink.gif)
Taking an interest in such a subtle detail must mean you really got into the game! I'm glad to hear that =D