Oh man, I have several paragraphs about this topic if you search the thread. And I just replayed from the beginning and there is some odd stuff around the "deaths" of the parents and sister. The parent's fighting all the time and the sister basically hiding from them often, meaning that she might not have even been near them when the plane went down if they had been fighting. It took over a year to find the plane, but the MC was told that there was someone "in his mother's arms" that must have been the sister. The kind of lie you tell a kid to give them closer and not because it is true. Not to mention that isn't how plane crashes work anyway. A year in the water and you wouldn't find two bodies holding each other. The water movements alone would have drifted them apart and fish scavenging off the dead would scatter them as well. A year on land and same deal, if it was a flaming crash then you'd be lucky to tell where one body ends and another starts, a year later and all you've got is a pile of burnt bones, if animals haven't dragged them off.
Though the parent's bodies were supposedly identified, anyone who has ever read a story, period, should be able to see the possible thread left open for the little sister's return. No body confirmed, means that character could come back. It's story craft 101. Every Telenovela, soap opera, romance novel series, tv series, any entertainment really has done it. No body means possibility of life. Hell, even JRR Tolkien and GRR Martin uses this trope. Elven relics and burnt bones prove nothing. Even in the real world this holds true. You can't have a murder without a body... I could keep going but I don't want to beat a dead sister,
Personally, I think the MC's sister is Polly. But, if I were a betting man, I would say that even Moonbox is undecided, more than once Moon has said that there isn't a lot of planning involved and over writing so it's entirely possible that Moon has left the door open and has yet to decide if the story will go through it. Not to mention that there are two and half years of story to fill in so something interesting has to happen eventually, and a return/reveal of the sister is a pretty interesting plot point. Especially since Moon has indicated there won't really be any more love interests added.
The basic point is this: there is nothing inherent to the story that would prevent her from coming back. It would not create any plot-holes or inconsistency to do it. It all depends on if Moonbox wants to or not. And unless Moon has said otherwise elsewhere that I haven't seen, the door is open.
I rest my case for the time being...