The first time it happens, Mycroft is eight. She hears her mother saying to a friend that Sherlock is quite old enough to be weaned, but doesn't seem to want to give up the breast quite yet, and then she frowns, because when she'd got a baby brother she'd looked in to these things and she remembered that the World Health Organisation had said children should be breastfed until at least the age of two. She doesn't want Sherlock to be deprived. She slips away from the garden party and goes up to the nursery, smiles at the eager welcome from the cherub standing in his crib and rattling the bars. He is always pleased to see her. She stands on a box and lifts him out of the crib, carries him to the rocking chair, and cradles him in her lap while she lifts her shirt. The baby nuzzles closer, latches on to her nipple automatically, clinging to her. He doesn't get any milk, but he doesn't mind, because it's nice, to be cuddled like this, allowed to suckle. Mycroft likes it too. She lets him suckle her for half an hour, switching breasts half way through, until he falls asleep with her nipple in his mouth.
She takes to visiting him twice a day, once in the morning and once at night, alternating the breast she allows him to suckle. Her breasts are barely developed, hardly more than buds, but Sherlock doesn't care. He likes being petted and cuddled by his big sister while he suckles her.
By her ninth birthday, they've established a rhythm, and Sherlock fusses and whines if they miss a session. She coos to him softly and strokes his curls the next time, lets him suckle a little longer, and he rests his tiny fist on her other breast as he snuffles and pushes closer, his little jaw working as he sucks. He can walk now, and climb up onto chairs, and their parents smile fondly as he toddles to Mycroft the moment she enters any room, climbing into her lap, wanting to be held and petted. It doesn't occur to them that he'd like to be cuddled by them too.
She tells herself she'll stop when Sherlock stops wanting it. She tries to bring it up a few times, but his little bottom lip wobbles and he looks at her with big sad eyes and she can never go through with it. He grows, becomes boisterous and destructive, and only Mycroft can get him to calm down. "Come here Lockie," she says softly, encouragingly, and he becomes meek and allows himself to be led by the hand to somewhere private he can nurse from her. She never calls him Lockie at any other time.
A little after her tenth birthday, when she has been suckling Sherlock for nearly two years, she hears a surprised little grunt as he nurses. It's not unusual for him to vocalise as he suckles, he makes tiny soft noises of contentment most of the time, but this sounds different. "What is it Lockie?" she asks gently, brushing the curls off his forehead. "Milk," he replies curtly, latching back on and suckling enthusiastically, kneading her small pert breasts with his little fists like a kitten against its mother. She gasps quietly, and cradles him, allowing him to feed from her. She likes the slightly prickly feeling in her nipples as he draws her milk out, she likes how happy her little brother is curled up in her lap, calm and loved. He suckles until the milk is finished and shuffles across to the other breast, closing his little lips around her nipple and sucking gently, humming as Mycroft strokes his hair and he swallows all the milk she can give him.
They indulge every morning and every night, and sometimes Sherlock comes to her to request it. He is allowed extra on his birthday and at Christmas, and gradually her milk grows thicker, creamier, her breasts swell with it and she has to buy nursing pads to stop herself from leaking when she hasn't nursed him in a while. It becomes a nightly ritual for them, Sherlock climbing into her lap and snuggling against her, happily feeding from her bare breasts as she pets him and croons to him, telling him what a good boy he is and how much she loves him.
Somehow, it never stops. Sherlock likes her milk, likes the intimacy and attention, and Mycroft cannot bear to say no to him, even when she grows older and understands the implications. When she is thirteen she tells him that they can stop whenever they like, and Sherlock whines and clings to her. He knocks on her door that night, half an hour before their usual time together, and clutches the hem of his pyjama top as he asks "Can I? Please?" in a tiny voice, peeping up at her through his eyelashes. "I don't want to stop, Mycie." She exhales and scoops him into her arms, sitting up against the headboard and lifting her top to expose her breasts, holding him to her. "We won't stop until you want to," she reassures him, even though he is six, even though he has started school.
If he's been good, she allows him an extra feed a day, just after he gets home from school. He behaves better if he knows he will be allowed to suckle from her. When he grows older, and the other children realise he's different, he starts to come home from school in tears, feeling rejected, believing their label of Freak. He clings to Mycroft and sobs into her blouse, and she hushes him gently,
Sherlock nurses from her long after he stops needing it. He simply wants it, he enjoys being given the only access to his sister's breasts, likes that her milk is all for him. He knows it is abnormal. He doesn't care. He outgrows her lap, slowly but surely, until one day he is all of eleven and Mycroft is eighteen. She comes home from nights out smelling of cigarettes and sweat and alcohol and men's cologne, and Sherlock grows surly and moody when she is gone, but she soothes him when she gets home. "Come here, Lockie," she offers gently, unbuttoning her blouse, and he regards her warily, until he sees that her breasts are swollen and full and untouched. He relaxes then, and takes his time nursing from her.
He realises when he is fourteen that he cannot have this forever. One day, probably soon, it will have to stop. He begins to act out again, calm only when he is on her breast and she is running fingers through his hair and cooing to him. It's what gives him his first wet dream. He grows unreasonably aroused by the thought of suckling his sister's breasts, how she opens her shirt for him and allows him to rub and suck them, her nipples plump and stiff in his mouth. Of course she notices his erection when he's nursing. She says nothing, but rubs his stomach gently. One day the pressure in his trousers becomes unbearable when he is halfway through one breast and he has to rub a hand over his aching crotch. Mycroft croons softly, replacing his hand with her own. "Let me take care of you," she whispers tenderly, the words nearly making him come in his pants. She unzips him and rubs him through his boxers as he continues to suckle, crooning to him. "That's my good boy. " It is over embarrassingly quickly. She kisses his forehead proudly and cleans him off with tissues as he finishes nursing.
They begin to share a bed, unbeknownst to their parents, so they can nurse last thing at night and first thing in the morning. They snuggle together under the blankets, limbs tangled, as Sherlock pushes up her nightie and nuzzles her breasts, rubbing and licking her nipples before latching on.
One day when Sherlock is fifteen, he finds his thigh between Mycroft's legs as he suckles, his erection pressing against her. He rocks his hips, gradually working his way on top of her. He is big for his age, and just as tall as she is; he is wiry but strong, his curls wild and his voice deep. Mycroft knows he is attractive. "I want you, Mycie," he whispers hotly, his thick cock pressing against her knickers. "Please." She strokes his cheek, opens her mouth to speak, but closes it again after seeing the lust and need in his eyes. She smiles and spreads her legs, lets him remove her pants, wraps her arms around his waist as he positions himself on top of her and moans softly as he pushes his cock inside her. He shudders at her tight wet heat, thrusting eagerly and clumsily, panting against her neck. In just a few minutes, he cries out into the pillows as he spills deep inside her. "I love you Mycie," he whispers, hiding his face against her collarbone. She pulls him up to look at her, his softening cock still inside her, and kisses him tenderly on the lips. "I love you too, Lockie," she says earnestly. He exhales sharply and kisses her again, growing hard at the sensation, and thrusts into her gently, spurred on by her quiet encouragements, moaning when she wraps her legs around his waist. He touches her breasts as he fucks her, making her gasp and moan. He always was a quick learner. He thrusts in as hard as he can when he cums inside her again.
It becomes part of their routine. He nurses first, eagerly suckling her milk from her swollen breasts, and then they make love, Sherlock marking his possession of her by cumming in her. He never tells her that she is his. He doesn't think he needs to.
When she moves to London, he turns to drugs. Their parents hide it from her. She has to buy a breast pump to relieve the ache in her breasts. She tries to call him, she sends their parents a key to her flat to give Sherlock, but he refuses to talk to her. His parents force him to go to rehab, and finally call her. She turns up on his sixth day in the centre, when he is weak and shaking, her eyes red from crying. He whimpers when he sees her, and turns away, ashamed. An arch eyebrow sent the staff from the room and she sat on the bed, gazing at him, her heart breaking. "Come here, Lockie," she whispers, opening her arms. He snaps his eyes up, lower lip trembling, and flies to her, throwing his arms around his neck. "I'm sorry, Lockie," she says shakily into his hair. "I'll never leave you again." He sobs against her blouse until he is calm, and unbuttons her with shaking hands, latching on with a whimper and nursing desperately, clinging to her as she redeems him with her milk.
She takes him to London with her when he leaves rehab. They find their old rhythm again, sharing a bed, snuggling quietly together as Sherlock suckles. She tests him regularly for illicit substances, telling him if he fails she won't allow him to nurse for a week. He never fails.
He finds consulting detective work and she becomes the British Government. Their sexual relationship expands outside of nursing, and he fucks her bent over the kitchen table or up against the wall in the shower. He loves cumming inside her, filling her with his seed; it becomes his new drug of choice. Mycroft has no objections. "You know what this means, don't you," he growls one day, pounding her into the mattress, watching her breasts bounce with each thrust. "Me fucking you like this, filling you with my cum. You're mine." She nods breathlessly in agreement, pulsing around him, and he fills her again.
When he decides he wants to take his possession of her further, he whispers in her ear as he's fucking her over the arm of the sofa that he wants her to carry his baby, to grow round and ripe with a dark-haired baby he put inside her. She gasps as her orgasm hits her. She doesn't go off her birth control right away, she's too clever for that; she has him give her sperm samples "for genetic testing" and arranges for embryos to be created using his sperm and her eggs. She selects two for implantation that are genetically healthy while Sherlock is away for six weeks on a case; one implants. When he comes back, she pulls him into the bedroom by his lapels and tells him to give her a baby right then and there, and Sherlock grins and tackles her onto the bed, fucking her with enthusiasm. He knows what's really going on, of course, but that doesn't make the game less fun.
Sherlock is thrilled when Mycroft confirms her pregnancy, kissing her heatedly on the lips. He watches as his sister grows heavy and round with his child, her ripe belly pushing out in front of her, the knowledge that it is his baby inside her arousing him almost unbearably. He worships her growing belly at every chance he gets, whispering sweet filthy nothings in her ear; "What must they think of you? So obviously pregnant, but without a husband or boyfriend… what would they think of you if they knew the baby you're carrying is your brother's?" She doesn't answer, only shivers lightly and presses into his touch.
Her milk becomes creamier as the birth approaches, and Sherlock loves it. He gains four pounds because he feeds from her so often, his gaunt look replaced by plumper cheeks. Mycroft cradles him as he suckles, just like always, and pets his hair. Sherlock curls in closer, wrapping his arms around her, humming softly as he feels their baby kick.
Mycroft gives birth to their son at a private hospital in Switzerland where nobody knows them, and Sherlock is there to hold her hand through all of it, to cut the cord and kiss her full on the mouth in his joy. When they can finally take him to their rented chalet he sits up against the headboard and holds them both against his chest as Mycroft feeds him, their tiny baby boy with dark curls and his mother's complexion. They name him Leopold Fitzwilliam Holmes, and they both adore him more powerfully than either of them thought possible.
When he is bathed and dried and snug in his soft fleecy sleepsuit, Mycroft croons to him as she feeds, the baby on one breast and Sherlock suckling happily on the other. She kisses both of their foreheads, smiling as her boys snuggle closer. This is how it was meant to be, she thinks, whatever we were to each other Sherlock has always been mine and I have always been his. The small, forlorn creature rattling the bars of his cot and pleading silently to be held and loved and petted had always held her heart, one way or another, and now their love had made another tiny person, who would never want for affection. She smiled and rested her cheek in Sherlock's curls, watching as their baby blinked up at both of them.