Yep, I think there is a possibility that it should be fine, IF she plays the paladin part, otherwise, B3
(broke beyond belief)...
Of course, why a half-succubus? There can be no buff help to the paladin so the half-succubus is really a hinderance (except for the high CHA bonus). And since she is fighting her nature, I'm sure the lords of Hell would be plotting her downfall at every turn, frankly a party aligned with her becomes pretty much devil fodder after a few sessions. If this is the entrire basis for the campagn thats fine I guess, but I don't know, I would be leary. Talk to your DM and make sure he's thought this through with his BRAIN not his nether regions.