I blame ~DuplexFields and his fanfic here: [link] for this scribble here.
After I watched "Sisterhooves Social" and noticed Dinky competing in the sister-centered competition with this unicorn pony (fan name Sparkler), I wrote it off as being her babysitter or maybe a relative on Dink's father's side. But I liked that little fanfic.
Such a sweet mom to both of them. She may be Dinky's biological mother, but she is truly the Mom of these two young mares. Anypony can be a mother, but it takes a mare of real caliber to be a Mom.