SpoilerEr, what? Why does this require Derpy to have a sister as young as Dinky? The pony Dinky was with is either Dinky's sister, in which case she's Derpy's daughter whether biological or adopted, or she's not, in which case she could be a friend, any different family member, or even a friend of Derpy's.
Unless Derpy was in the contest with someone we've never seen, which I do not recall being the case, there's nothing here suggesting that Derpy has a sister as young as Dinky.
Zevox