This is my first headcanon, so hopefully this is the general idea? Also this got longer than I planned, and includes a little Arya/Gendry. Hope you like it!
It’s an old, outdated tradition, that girls see the surname of their soulmate on their arm once they’ve flowered. There are stories that men used to see names too. For men it was different, a woman’s first name appeared, not her last name, inked onto a man’s arm when he reached puberty. But now only girls bear the mark. The marks are respected secrets, and a girl can close her eyes and choose to let another see the golden script on her arm.
Sansa loves the custom, though, and thinks it will be one of the best parts of growing up. It’s a small consolation to see “Targaryen” appear on her arm when she bleeds for the first time. At least it’s not Joffrey’s last name, but she already knew the gods would be cruel to make that monster her soulmate. And besides, what Targaryens are left?
The tradition causes a world of trouble for bastards, because only a man’s true surname will bloom on someone else’s arm. Somehow, the tale goes, the gods know your real parentage. This also means “Stone” and “Waters” and “Snow” are never words a girl sees on her skin.
Sansa falls in love with Jon when she still knows him as a half-brother. She’s surprised at how little guilt or shame she feels. She’s numb to the judgment of others, and coaxes him into her bed regardless. She feels a throb on her arm the first time they are together, and recalls later that was in the soulmates stories too. But the night she spent with Jon was so beautiful in other ways that the detail falls to the back of her mind quickly.
When Jon’s parentage is revealed, Jon’s dumbfounded, then obstinate. He refuses to believe it’s true, because he’s wanted to be a Stark for so long. The evidence is overwhelming, though. Others encourage him to embrace it, and deal with the implications, but he won’t. After impatiently listening to fifteen minutes of argument, Sansa takes Jon into her chambers and shows him her mark, willing it to appear for Jon’s eyes. This has always been a woman’s privilege, done in private, and many married men know they will never see their wife’s mark. Marriages are rarely made for love, most often for political considerations, and it’s the stuff of songs when love and politics coincide. Jon traces the name in wonder, and the next morning, in a remarkably good mood, acknowledges his heritage.
Sansa and Jon are thrilled to see Arya make her way back to Winterfell, though there’s a distance and anger in Arya still. She’s not inclined to talk about it. A few days later a young man named Gendry Waters shows up looking for work, and Jon gratefully accepts. Their ranks have been decimated, and they could use a good smith. Sansa catches Gendry glancing at Arya longingly under his dark eylashes, and doesn’t miss the pointed way Arya avoids him. Obviously they know each other. If Arya’s in love, Sansa thinks, it would likely manifest this way. She’s not sure how to talk to Arya about it, so she starts with her own story, and shows Arya her mark. Arya sighs and tells Sansa she’s happy for her, really, because Sansa deserves an ending like a song, but what is she supposed to do with “Baratheon” on her arm?
The mystery of Gendry’s birth is sorted out, and Arya, with a roll of her eyes, relents to Sansa and Jon’s kind advice to talk to Gendry. She bares her arm and shows Gendry her mark, right there out in the open in front of the forge, glaring at him, daring him to deny it. Gendry drops to his knees instantly, and asks Arya to marry him. When Arya says yes, so quietly Sansa and Jon have to strain to hear it, he picks her up with a shout and spins her around. Arya’s covered with dust when Gendry sets her down gently. She’s beaming, the first real smile Jon and Sansa have seen since she returned to Winterfell, and Sansa thinks some traditions might still be worth keeping.