Looking for a story · 7:19am Nov 15th, 2019
It may have been deleted because I can't find it. It was an EQG story where Flash Sentry and Rainbow Dash are brother and sister but they didn't know about it and they were the result of a ménage à trois. Scootaloo is also their younger sister.
Sounds interesting, can't say I've seen it though...
https://www.fimfiction.net/story/219801/117/equestria-girls/a-difficult-talk
You'll find it somewhere in this story. David was asked by the mods to compile most of his one-shots into anthologies.