Flash Sentry, Sunny Delivery, Fluttershy, and Zephyr Breeze are descendants of Flash Magnus, which makes Blue Moon, Comet, Super Scoop, Flash Flood, Hullabaloo, and Hot Spot descendants of him too! When Flash Magnus was rescued from Limbo, he wanted desperately to get back into a normal life, which for him included a career and a family. Rather than immediately jumping into making one of his own, he searched for any existing relatives, no matter how far down the line they were. He was ecstatic to find his descendants, with Flash Sentry and Sunny Delivery knowing of their ancestor but Fluttershy and Zephyr having no clue until they researched their maternal family line.
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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.