(Anthro Pony Type)
Rainbow Dash was an only child. A long time ago on one Hearts Warming Eve, her mother had miscarried the one who would have been Rainbow’s baby sister. However, on that night, the young Rainbow Dash knelt down in front of her Hearts Warming Eve tree and offered an earnest prayer:

"Please make my mother well again, and please give me a little sister.”
Years have passed since the lonely Hearts Warming Eve, and Rainbow Dash has nearly forgotten her prayer. Oh just you wait Rainbow, for Santa Hooves hasn't forgotten. It was Hearts Warming Eve once again, and Rainbow was excited for the presents, the parties, and being with her friends. Rainbow Dash headed home after Pinkie’s party only to be greeted with the best unexpected gift of her life… Her sister.

(Authors note: I'm sorry about the last part. I'm just reading, and watching some other things at the same time. Also, I got completely off track from the matter at hand. Again, I’m sorry you guys. Please forgive me).