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In my verse, Flash Sentry and Sunny Delivery are half-siblings! Fluttershy and Zephyr are distantly related to them, like great cousins multiple times removed.
I think Flash Magnus is actually quite a bit shorter than ponies think (and technically younger than his descendants from his years stuck without aging in Limbo), especially since his story details him taking on a dragon. I’m sure anyone would imagine him as some huge, strong pony. But he’s meant for speed and agility with a smaller frame. Flash Sentry, on the other hand, has plenty of earth pony genes and Fluttershy/Zephyr Breeze are just ungodly tall.