Losing Sunlight

When Princess Celestia comes down with a mysterious illness despite her immortality, her loved ones flock to her side, including Sunset Shimmer. With only so much time left, Sunset takes it upon herself and Twilight to get to the bottom of it. The two of them work together non-stop to find a cure, which would be fine, if Sunset had gotten around to telling any of them she's dating the other world's Twilight.