He never verified but from what he could tell Jack came from some level of money. Sean Don didn't ask about these things; he personally came from a poor background. Even when he told people where he had grown up they would wince and say wow you're from there? That's the DEEP SOUTH. Sean didn't like the way people looked at him when he told them where he was from. So he chose instead to focus on the fact that he was a grad student.

Sean Don would never ask the literal details about Jack's family. Sean Don just interpreted based on observable facts. Twins in college at the same time. Jack seemingly paying for things without worrying about the amount. The fact that he wore nice clothes. Also the fact that he upheaved himself and went to England for weeks. That wasn't something a non rich student could do was it?

These things bothered Sean Don. Unfairness in the world. However he also would prefer to strive to be like Jack's family rather than his own. This meant he was jealous. This bothered Sean even more.

While Jack was gone across the pond Sean began to dislike Jack more. Jack became less of a human and more of a cultural symbol representing inequality. Sean knew once Jack returned things would go back to normal. But in the meantime he was dreading the return.