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".

Note, Fluttershy's brother is NOT among the rank of the Alicorns, but part of ordinary Pegasi.

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.

Finally, after "Flutterbro" marries Twilight, he forsakes the title of "Prince" (even though he is technically part of Royalty in Equestria) in favor of "Citizen".