Serah isn't just Lightning's sister; she's the center of Lightning's entire character.
The only reason Lightning involved herself in FFXIII's plot was to save Serah. The only reason Lightning was the angry hostile woman we played as instead of the sweet woman we saw talking to Amodar in that one flashback is because Serah dated a bum and got turned into a l'Cie.
If she doesn't come back then Lightning just will not end up happy like Toriyama wants her to be. So unfortunately we'll have to expect an asspull Serah return.