Not much I guess. Perhaps too bothersome, only for the novelty...
I had tought on the birth notice notifying the birth of twins.
Then twin boys would be sold, perhaps for double the money for being two people, or a bit more if they were sold together...
Twin girls would be two girls for the time you had to wait for one. Maybe they wouldn't run away without the other...
Not much effect, game-wise, I guess... But then I tought, (exaggeratedly and optimistically) that lesbian sex between twins would be the famed twincest...