Two Brothers, One Crew, and the Sky

AU: Sam and Dean are two homeless kids from the planet Persephone. They had finally "made" enough money to get off the planet and go someplace new. With adventure in mind, they stumble upon the ship Serenity. They planned to get off on a new planet, but like they say "There's no place I can be, since I found Serenity." Now, if the crew'll have them, they plan to keep flying.