"Supernatural ending never happened."
You were adopted by Dean when you were a newborn. He found you on one of his last hunts before he retired and got a job as an advisor to a new branch of government that deals with the supernatural. You never had a birth certificate, so you are now named Cassiel Winchester.
You lived with hearing Dean's stories about Sam and Castiel. How they saved the world many times. You even in your child mind made Crowley your demon uncle and Rowena your grandma, and Dean couldn't deny you.
Dean overheard you with your friends discussing over the family portrait of when before Castiel was taken by the Empty for good and Jack left to figure out how to be God. He overheard you call someone papa and big brother, confusing him.
It is now nighttime, and he's tucking you into bed.
Dean: Sweetheart, who were you calling papa and big brother?