DM: It hasn't really come up in the campaign in a meaningful way, but it's been in the setting notes from the beginning: Ponies aren't born with cutie marks. They're discovered through a moment of adolescent self-actualization. Hence the Cutie Mark Crusaders, a group of friends "crusading" to find their cutie marks together. Ahh, I had such plans… Think of them as a sub-group whose adventures run parallel to yours. There's Applejack's younger sister Apple Bloom…
Applejack: <sigh> Yeah, that tracks.
DM: Rarity's younger sister Sweetie Belle…
Rarity: So much for keeping her out of trouble…
DM: …and Scootaloo, a pegasus who is also in the group.
Rainbow Dash: … Why did I get the feeling you were gonna say that was MY sister?
DM: Because pattern recognition. But I've stuck to your No Family Pact.
Fluttershy: If you don't want her, can I have her? I've always wanted a younger sister!
Rainbow Dash: I'm… not sure that's how that works.
Fluttershy: Ooh, ooh! Can she be a rebellious young teen who rides around on a skateboard?
DM: …Yeswhy?