Because no fandom is ever complete without a bit of sibling-cest. I may continue this story later should inspiration arise but this story can be read by itself.
Robin is far too young when she meets the love of her life. She's twelve, still too much into rock climbing and hunting to be classed as a proper girl, and cries herself to sleep every night thinking of her mom and sister living somewhere (anywhere) without her. So when Robin Senior, she stopped thinking of him as her dad years ago, introduces her to his new wife and her son she doesn't pay attention to the butterflies that roil up in her belly just at the mere sight of the yellow haired boy with sky coloured eyes. Instead she yells out a curse word or three in French and threatens to shoot them if they don't get off her property.
Robin Senior predictably doesn't take that well but instead of slapping her down or, worse, punching her out he just laughs and ruffles her hair a little. She's still grounded, still has to singlehandedly carry all the luggage upstairs from the car. And she still has to share her pokey little bedroom with a sixteen year old boy she absolutely refuses to think of as her brother.
"Don't worry kid," Barney tells her much later that night once they have changed into their pyjamas and, for Robin at least, pretended to brush their teeth. "I don't intend to stay here long. You'll get your room back in no time."
"I don't want a brother," Robin grounds out by way of reply and Barney rolls his eyes, thumps his pillow twice, and switches off the nightlight before settling in for the night.
It's a further two weeks before Barney realises that she is a girl and consequently demands his own room only to have the request shot down in flames by both of their parents. The spare room is for guests and the basement is off-limits to anyone whose name doesn't end with 'Senior' and so they share a room and a closet and a bathroom for years.
It is Barney who notices when Robin needs a bra after a surprising growth spurt three months into their living arrangement, Robin who catches Barney jacking off whilst phone-sexing his girlfriend back in New York, and Barney who changes Robin's sheets when she starts her period the morning of her thirteenth birthday and subsequently goes into a teary kind of shock that is apparently normal amongst menstruating females.
However two years into their arrangement it is Robin who peels off the thin Star Wars t-shirt that she uses as a nightshirt when she catches him looking at her a beat too long. He has just gotten off the phone with Shannon, apparently she tripped and fell and yes it is possible to accidentally fuck someone whilst waiting for your boyfriend to return from his Canadian exile. Barney is devastated and Robin feels confused (and powerful) and before either of them can blink they are naked and panting and wanting under Barney's sheets.
The next morning Barney leaves never to return and a year after that Robin follows him.