Age is wrong. Sorry!
The thing is, Melissa is "probably" dead. All this may be just a mirage. I just can't deny that it is a plot point that is extremely attractive. Not only because the MC recovers his sister, Nana recovers another grand daughter, Amy & Liz a niece and Cat a very close cousin. It also means that the tragedy of the disaster wasn't complete, that there's still a small chance to reduce the harm.
This, is all probably futile and it is all based on what could very well be on a wrong interpretation of the disaster description. It may not even be on MB's intentions to launch a mystery. YET. I can't shake this impression. I simply can't. And the fact that I don't know why, is what irks me the most.
Peace