If you are in need of this niche, I got a story for you!
Sister's Gift
With his parents gone for the week, Micheal looks over Jessica, who has only one wish, and she gets it, in form of a gem, and a purple diminutive equine.
Features: Pony TF/TG, mental changes and a whole lot of "darling".
This story would have not been possible without TF-Sential, who is your source for quality Pony TFs. Seriously, he is amazing! Also, the story has a accompanying TF sequence, if you are more for the visual side.