Actually, Cadence was Twi's foalsitter BEFORE Twilight got into Celestia's school for gifted Unicorns. At least, that's what I gather from the show, a Canterlot wedding showed that Twi didn't have her cutie mark at the time, so this makes sense in that respect. Also, in the comic "The fall of Sunset Shimmer" shows blank flank fily Twilight with Cadence and Shining Armor