I felt like writing something dark for Zoe and Mal; the idea of Serenity as something you can never escape haunted me. Also, the fact that Mal used to believe in God and never, ever mentions his family.
He had a twin brother once, but he doesn't tell anyone. Just like he doesn't tell the rest of the crew that not all Reavers come from Miranda (that the very, very first came from somewhere else).
Zoe knows this but keeps quiet; doesn't blink at his lie of omission.
Sometimes he thinks River might know, or at least have an inkling. It's hard to really know when you haven't been there yourself. It's hard to understand exactly what Serenity Valley was (and still is, eternal and always).
She knows hardship and loss and surviving beyond all costs, but she still has her brother. She also probably has a halfway decent bit of soul rattling around in her head. The only reason she suspects is because she's a reader. River knows that there's something not quite right; that his (and Zoe's) thoughts are much quieter than they should be. (still waters running so deep and dark and black)
The good Sheppard Book might also have an idea. He was old enough to have fought in the war (and be on one of the retrievals ships sent to pick up the survivors of that particular conflict), but Mal knows he didn't fight there. By the end there were only a handful of them on either side and they wouldn't be forgetting the faces anytime soon. Ever. Captain of a drop ship would make sense; or even commander of those fancy crack black operation troops they sent down to clean up the big messes (or make them, depending on the need). The sight (reality) he would have been met with in Serenity Valley was probably more than enough to send him packing to the nearest abbey. Most didn't even make it that far (7 suicides, 13 cases post-traumatic breakdown, 9 MIA at last count).
Faced with these kinds of numbers, Mal is understandably reluctant to tell his wonderfully semi-innocent crew some of the meaner truths of that war (the ones that go way beyond 'war stories'). Like the ones about his twin who was also a chaplain; about how they were left for four months among the diseased and dismembered corpses of their friends (and enemies and perfect strangers and children). About how the very, very first Reaver was born when one of their own was driven beyond madness (because they were all a little crazy then, but what happened to him was something new). Mal will not speak about his brother, who is lost to him now, just as he will not speak about his God, who is as equally dead. He will not tell them that not all Reavers mutilate themselves so thoroughly.
Some are made understanding a more profound kind of pain. Some know that they can do so much more damage when they wear the face of a friend. Some (one) is a demon in Sheppard's robes.
The thing that wore his brother's face emerged from that planet like a grotesque butterfly. First came the news that a medical transport had found one more survivor among the dead. Then came the news that the medical transport was missing. They had figured out who the 'other survivor' must have been by then and didn't bother asking if the transport had been recovered. Five months later and word has it that a couple of old retrofitted cargo haulers dropped of the grid around the small mining colony on Lauron. Then Lauron followed suit. By then the stories had started. The lines between truth and boogeyman had blurred.
To be honest Mal is a bit surprised his crew never picked up on the little discrepancies.
Reavers were beyond reason, true, but that didn't mean that they weren't smart, that they weren't organized, that they didn't have a leader. They had a fleet, god damn it. Where, on Miranda, they had only mindless rage and aggression (and would have destroyed themselves eventually), there were now ships and pilots and weapons and direction.
And wasn't that clever; make people (innocent people) into one of them by making them watch and understand and survive just like his brother did in that valley all those years ago.
Mal and Zoe are not the people they are on the surface of their placid minds. They are, and always will be, the fatally flawed children of Serenity Valley (just like all the others; just like him).
The family they have carved out for themselves don't understand what that means. Not really.
They don't know. They have no idea. And with any luck, they never will.