@Shepherd
I think the list of Ponies with siblings is shorter.
Celestia/Luna
Twilight/Shining Armor/Spike?
Applejack/Big Mac/Applebloom
Rarity/Sweetie Belle
Fluttershy/Zephyr Breeze
Pinkie/Maud/Marble/Limestone
Thunderlane/Rumble
Moondancer
Sky Stinger
Gloriosa Daisy/Timber Spruce
Flitter/Cloudchaser
Aloe/Lotus Blossom
Yona
Sandbar
Ocellus
…
That’s everyone I can think of.