Cuz frankly I aint sure. I mean, one could say that this would only make SENSE due to the fact that she's Lightning's sister, but from the looks of the game's ending (which I saw on Youtube) it looks like Square's trying to avoid the whole deus ex machina routine.
Honestly I think that things are getting way too unpredictable with these guys. I know I'm not the only one who was caught off guard by the second game's focus on time travel.. --- "In the hands of someone who's good enough at thinking outside of the box, ANYTHING can be used as a weapon."
They'll bring her back... during the last 2 minutes of the game like XIII. They said Lightning gets a happy ending, and Serah being alive depends on that. But she won't come back any sooner than the very end. They can't resist adding in two trillion tons of melodrama and angst. --- "Mahroflcopter... mahroflcopter... mahroflcopter goes squa squa squa!" Now currenly searching for rofl to fuel mah copter.