Disclaimer: I do not own Fullmetal Alchemist or any material related to Hiromu Arakawa in any way or form.
Note: This was inspired by SeeInBlackAndWhite's fic entitled "Beautiful Tonight". Many thanks for that inspiration.
There's never any warning for Fletcher when Russell has his 'time of month'. But it isn't the time of month that so many girls his age endure. No. His monthly cycle is when he gives in to the pain, fear, shame, and resentment of being his father's son.
Before Xenotime, Russell had had only one goal in mind: to continue Nash Tringham's work researching the Red Water. Even if it had only been a cheap imitation of the Philosopher's Stone, it had been important enough to Nash, and to the military that supported him. Both Russell and Fletcher had been too young to understand why their father had left them and their mother one day and never returned. But after their mother's death, Russell tried to fill Nash's shoes and sought a way to further his father's research, eventually ending up in the gold-and-agriculture-bereaved town of Xenotime, where he was taken in by Mugear's glib words.
After meeting Edward and Alphonse, the Tringhams' eyes opened to reality. They realized that by continuing the research their father had abandoned, it had caused an epidemic to resurface, one that had reaped a harvest of infant lives in years past. A whole generation of children. The memory of Mugear telling him that still made Russell shudder in horror.
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Fletcher watches his elder brother with a pinched face, as though he too were in pain. But his pain stems from the sadness of watching his strong role model break under the strain of living with the shameful things he and his father have done. The crimes they have both or nearly committed, such as Mugear's unthinkable notion of introducing Red Water into the drinking supply of pregnant women. Fletcher knows the extent of his brother's relief that neither he nor Nash had gone that route. But he also knows that Russell is still haunted by the thought that whatever blood their father had on his hands, he is also forced to bear.
It is during these times that Fletcher knows Russell needs him more than ever. Every month, his elder brother succumbs to his memories of the horrors in his life. Fletcher is the sole witness as Russell curls under the sheets at night in a cold sweat, paler than death, skin clammy, eyes dulled with lifelessness, voice mute. It is during these times that Fletcher must be the responsible adult and help Russell recover.
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Russell knows what he does isn't fair to his brother, but he's too wrapped up in selfish fears and memories to care for longer than a split second. He knows he should be getting help, therapy, a girlfriend, anything to stop himself from falling further into the spiral of negativity. Russell's every waking thought is of self-loathing, resentment for his father, silent begging of forgiveness from Fletcher, and gratitude that the Red Water was destroyed by his younger brother's alchemy. And when he dreams, Russell's every fear becomes reality, nightmares, until his mind nearly splinters from the horror of it all.
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During his time of the month, Russell wallows in despair; horror and hatred twine themselves around him so tightly they consume him, suffocating him until he's at the brink of insanity. Fletcher is left alone, in vain, to console him; to try and bring the light back into his elder brother's eyes, the carefree, cocky attitude he once had. And despite all the hardship—the horrors, the nightmares—they stick together.
Because that's what brothers do, right?