This story is an Alternate turn of events from the Episode Brotherhooves Social

Applejack was called away once more to go on another epic adventure with Twilight. However, Big Mac is once again left behind, except...this time, the Sisterhooves Social is coming up, and Apple Bloom is without a sister to compete with. That is until Big Mac decides to recruit Zecora into brewing a gender-swapping potion and Rarity to help him act like a mare. Big Mac is then transformed into Auntie Orchard Blossom, a plump, happy mare who visits Ponyville for three days to compete in the Sisterhooves Social. But during her short visit, she must keep her true identity hidden from the judges and also from Apple Bloom.