Wangxian Sabrina AU. Not the teenage witch stuff the 1954 classic that got a remake in 1995.
Wei Wuxian as the son of the Jiang family chauffeur who climbs trees to sneak a look at the fancy parties the family hosts, especially whenever the wealthy Lan clan attends. Sighing over the very handsome Lan Xichen, who is older and cooler and extremely pretty and just like prince charming, always being a gentlemen and dancing with anyone who asks.
Wei Wuxian also getting into arguments with the younger Lan, who keeps finding him up trees and lecturing him on improper conduct, and who never dances with anyone and always seems to have a frown affixed to handsome young face.
Wei Changze starts to worry that his son is missing out on his own opportunities by just supporting others and daydreaming about his prince charming, so he sends him off to see the world and attend a foreign university and hopefully find his own path. It works, Wei Wuxian excels in various fields (is there still cultivation in this setting? could be!) and comes home a few years later all grown up and magnetically handsome, oozing bad boy appeal without actually being all that bad.
Lan Xichen, meanwhile, has been locked in a string of doomed relationships with various unsuitable men for the past few years, to the point where Lan Qiren has put his foot down and arranged a match between his noble yet romantically disastrous heir and a respectable guy from one of the major sects. In the immediate wake of this Xichen goes to pay a visit to the Jiangs and sees Wei Wuxian, and starts asking around about who this might be because he seems oddly familiar but surely Xichen would remember a person like that.
Lan Wangji, getting wind from the Lan servants that his brother might be picking up yet another disastrous liaison in some uncharacteristic but not improbable response to their uncle's mandate, starts scrambling for a way to prevent catastrophe. Doubly so when he finds out that this new beau is Wei Wuxian, the devastating seducer who has been capable of stealing anyone's heart and crushing it to dust with just a wink and a laugh since they were mere teenagers.
The path forward is clear -- Lan Wangji must seduce Wei Wuxian before his brother falls in love with him and messes up his future!
Of course the plot twist in this case is that Lan Xichen wasn't actually angling to seduce Wei Wuxian, and makes no further overtures in that direction, being more like "aw he grew up nice that's swell", and Wei Wuxian got over his crush ages ago, but Lan Wangji is committed. To do right by his family he simply has to convince Wei Wuxian to fall in love with him instead of Lan Xichen. Then marry him, because he's not a monster obviously if he's going to seduce someone he will take responsibility for it. It's the only way.
Shush, Uncle, it is certainly not 'unnecessary' or 'completely absurd' or 'not a rational line of thinking at all'. It's the only way.
