Pegasi don't have mothers. They reproduce by fission... like all bacteria (c)

In all seriousness, in any cannon material there is never any mention of parents of any pegasi... well at least none I can remember. Fluttershy and Rainbow seem to be living on their own, none of their parents are ever mentioned, and there is, I believe, a whole subgenre of Scootaloo adoption stories, where she is believed to be an orphan of some sort.
Of course none of Mane Six parents are mentioned in any detail, but we at least get the names and appearance of Twilight and Rarity's partents, so they definitely exist. It is also likely that Applejack's parents are dead, but we see Pinkie Pies whole family in season 5 (even though I still say she is adopted), so the specific lack of any mention of Mane Six (or, in fact, any other) pegasi parents is somewhat unusual.

So, I have a theory to explain it.
Namely, from the Hearthwarming episode we know that it is likely that pegasi had (prior to Unification Era) a highly militaristic culture, not unlike Roman or Spartan. Therefore I believe that perhaps in pegasi culture - there is no concept of nuclear family.
Rather than raise their foals with their birth parents, they are likely either raised in special institutions, or, more likely, are raised communally with every foal being watched over by any available adult, which would explain how Scootaloo gets to run around all by herself, with special institution (like Flight Camp) taking over in teaching them basic skills that would usually be covered by family.