AU where Lan Wangji picked up Wei Wuxian's ribbon as well as A Yuan when he came to the Burial Mounds after the siege. He washes it and takes really gentle care of it until Lan Sizhui is old enough to get a forehead ribbon and he gives him Wei Wuxian's old ribbon instead of the usual one and tells him that it's from his mother.
The other Lans are very confused when Lan Sizhui shows up to class wearing a red forehead ribbon, but accept it once Sizhui tells them its his only keepsake from his mother. Lan Qiren splutters and goes purple at the sight of him that day, but he never hears anything else from his Great Uncle about the ribbon.
Lan Sizhui wears it and has to explain why its red about a billion times until he stumbles upon Mo Manor where Mo Xuanyu starts mumbling about him wearing his old ribbon.
Wei Wuxian works out that Lan Sizhui is A Yuan a lot sooner and there is a very tearful reunion.
