This One Time With Julia by David Lampson

After Joe's parents died, he stopped growing up. He doesn't know where his money comes from. His diet consists primarily of cheeseburgers from McDonald's. He plays basketball on the level of a pro, but he has only ever played on the streets. Then his brother disappears, and Julia shows up. Joe falls in love with Julia as quickly as his twin brother, Alvin, did. And like Alvin did before him, he runs away with Julia to her parents' hotel. There, he's so blinded by her seductive, dysfunctional family that he can't see the truth of his brother's disappearance ...until he accidentally stumbles upon Alvin's killer.