Fluttershy's brother will likely make his debut in MLP Friendship is Magic in season 6 in 2016. He moves to Ponyville and becomes close to Princess Twilight Sparkle. They exchange valentines on Hearts and Hooves Day, and eventually, marry each other in the two-part final episode of season 6.
I also presume that Fluttershy's brother is her older sibling, and that he was a mentor to his sister and to Rainbow Dash, and called them "Shy" and "Dash".
I also guess that the mother of Fluttershy and her brother is a member of Cloudsdale's ruling council and possesses the title of "Lady". Also, I predict that Wind Rider , featured in the episode Rarity Investigates!, is the father of Fluttershy and her brother, and Wind Rider divorced his wife sometime after Fluttershy was born.