Diamond Mine has always wanted to participate in the Sisterhooves Social. With her Mommy and Auntie Rarity participating every year—along with her friends’ moms and aunts—she finds herself on the sidelines, once again longing for a sibling of her own. It doesn’t help knowing that her friends with brothers could theoretically participate as well, since the event is technically open to any and all sibling pairs. Being an only child is lonely…  
One year, Diamond notices another pony on the sidelines: Chocolate Cheesecake, who, while not an only child, stays on the sidelines to watch his two sisters race together. They start talking when Cheesy admits that he’s secretly always wanted to join the race, though he lets his sisters have their fun. Diamond takes this opportunity to tell him how she’s always wanted to participate too.  
With Cheesy being Ponyville’s resident Drag Princess, one thing leads to another and he and Diamond make a plan to participate in the Sisterhooves Social together! They both don matching dresses and Cheesy dolls himself up (though everypony knows it’s him), and they run the whole course like they’ve always wanted. While they don’t win, they end up having the time of their lives anyway. This becomes an annual tradition for the two friends that continues for years to come!