I read this book several years ago. All I remember about the plot is that teens (who have special abilities) are part of an organization run by gods. There's another group that is trying to defeat/usurp the gods. The main god has two sons. One is mortal but has been given the gift of immortality and the other, Matt I think, is naturally immortal. There is a love triangle between Matt, his best friend (also a member of the god's organization), and Rachel, who betrays them to the evil-doers. Matt's sister ends up falling in love with the mortal son of the god. There is some time travel. I know it's a long shot that anyone has heard of this novel but if you could help me out I would be eternally grateful.
As per Nico's comment above, this is apparently "The Named" by Marianne Curley, the first book in the The Guardians of Time trilogy. Ethan and Isabel are the main characters. Isabel's brother, Matt, Ethan's former best friend, isn't introduced, however, until the second book.
Ethan lives a secret life as a Guardian of the Named. Under the guidance of Arkarian, his mentor, and with the help of Isabel, his unlikely but highly capable apprentice, Ethan has become a valued member of this other-worldly corps. As the only defense against the evil Order of Chaos, the Named travel through time to prevent the Order from altering history and thereby gaining power in the present and the future.