That's the thing: we're told her body was never found. And the game even brings this up at one point, with the mc wondering whether his sister survived. I mean he briefly entertains the idea that a girl (who is clearly of a different ethnicity to him) could be his sister. I'd call that loading a sister-shaped gun with a round of potential incest and putting it on a wall.
In writing terms, it's called a "Chekov's Gun". It goes something like this, If the writer mentions a gun, it will be used later in the story, in whatever way.
In this story, the gun is Melissa. If it wasn't to make her be alive, there was no need to mention the absence of her corpse. Moonbox knows this. So, it is still his prerogative to decide where or how to use the gun he loaded, or even if he uses it, while that is considered bad form, it is still within the realm of possibilities.
So, all we can do is: #SaveMelissa
Peace