My Little Pony: Friendship is Magic - By The Allsparks, It's Sweetiebelle's Parents.

Just noticed something. Everyone in Rarity's family have three pronged cutie marks. The father has three footballs, the mother has what I believe are three flowers, and Rarity having three gems. Maybe if they ever reveal Sweetie Belle's cutie mark it will be three of something too.

Oh so they are unicorns.