For a long time Sunset Shimmer thought she belonged in the alternate world, but she misses her home, she misses Equestria. Upon returning Celestia, Luna and Cadance welcome her to stay, Twilight isn't so sure. On one night of Sunset's stay, Celestia visits her. She tells Sunset Shimmer her worries about the Princess Of Friendship. Celestia then visits Sunset every night. One fateful night she asks Sunset if she wants Twilight's crown, and not to take back to the mirror world.
(There are not ANY Sunselestia shippings. The only romance here is between a guy and a girl.)