Yeah, Carver's a Templar for, what, six years by the end of the game? With no suggestion of "selling Hawke out".
Actually, that could have added an interesting element to friend/rival on siblings - rival Templar Carver tries to apprehend Hawke some time in late Act 2 (or maybe early Act 1 - given what else happens in late Act 2... Though arguably it could be made to work - furious rival Carver blames his sibling, distraught friend Carver reaches out to help his only remaining family* etc), friend Templar Carver tips them off about another Templar coming to apprehend him, rival circle mage Bethany runs away and becomes a blood mage, friend circle mage Bethany lets Hawke know about a group of blood mages meeting in Kirkwall... Dunno what the GW equivalents would be, mind, and obviously Carver is much harder to friend/Bethany is much harder to rival, so that might not be ideal balance-wise, maybe make the rewards better for friend Carver's fight and rival Bethany's, or whatever.
*well, okay, there's Gamlen