I don't know, Sheryl seems like the one girl of the two that can pick herself up, let Alto go and be happy without him and out on her own. She seems like too much of a strong character to be all wimpy about Alto lately and she's more than fine without him. I don't see why she can't just forget about him and just pilot the shit out of some VF instead.
Ranka's the one that though eventually she'd be able to accept that Alto would choose Sheryl, she'd have a much harder time being the "loser" and it seems like the one who needs a romantic interest much more in the end. Sheryl just seemed like the lonely songstress who just needed friendship and she's seemed to have found it since staying on Frontier.