So, what we're seeing is part of the joy of reading a story that's still in progress. Presumably, at the start of the story, Moon thought he'd leave the possibility of bringing Melissa in as the long-lost sister open. In fact, the way things are stated is a classic I first learned while watching soap operas back in the 1980s - if you want to let other characters move on for now, but still leave the door open for the actor to return in that same role later, have them be presumed dead, but without an actual body found.
Later, Moon (from what he's told us) decided not to bring Melissa in. (To avoid spoilers, let's say he may have found another avenue for a relationship with a sister). Again, based on what he's told us, that idea is done. If there's any thoughts of polishing the story once it's completed, he may remove the reference to Melissa's body not being identified, as it would be unnecessary and misleading.
Of course, a year ago we were being told there were two characters that were never intended to be on-going characters that we'd see again at the end of the story. And one of those two characters we saw in an update from (I think) late last year. So, just like leaving the option for Melissa to show up, Moon can still change his mind about what he hasn't written yet.
Personally, I think we are far enough into the story, and Melissa would be a major enough character, that she either isn't going to show up at all, or she'll show up at the very end, with the possibility of a relationship with her to be explored in some sort of sequel. Given Thomas' relationships with every other living relative he has, it would be odd for her to come in and not become an unavoidable character. Given the degree of trauma and separation the two have experienced, and the fact she hasn't been around the hypersexual Thomas we know, I can't see her just impaling herself on Zeus the minute she walks in the door. The story would almost have to become focused on just the two of them to do things justice at that point. And the fans of Allison and Amy would
not be happy about that. So, from a story structure appearance, if she does show up, it should be at the last minute, and there should be a sequel focused on Thomas balancing the needs of his existing harem (and children!) with his burgeoning relationship with Melissa. I can imagine something where he tries to insulate Melissa from the rest (with their agreement, at least mostly), leading to scenes with 5 naked women winding up hiding in the closet while they silently cheer Thomas on in getting Melissa to open up. Still a lot of comedy, but a bit more farce, and something that might on the surface look to be NTR (but nothing is actually hidden from the harem, and they're totally encouraging him).
(Hey,
MoonBox , if you like the idea, it's all yours!)