It's already been confirmed via Twitter that Fluttershy's male sibling, first mentioned at this years San Diego Comic Con, will NOT appear in this year's (fifth) MLP season.
That being said, Flutterbro likely will make his first appearance in next year's (sixth) MLP season.
I figure out that Fluttershy and her brother were both born in Cloudsdale as fraternal twins, but Fluttershy's parents divorced when she and Flutterbro were still foals. Fluttershy remained in Cloudsdale (I assume) with her mother, while her father and Brother moved elsewhere in Equestria, possibly to Baltimare or Las Pegasus.
That might make a lot of sense: having a pony of the Mane Six with a messed-up family life (thanks to having her parents divorced when she was young).
Add to that the possibility that Fluttershy's mother married another stallion, and had at least one more child thanks to her union with her second husband. Having a stormy relationship with her step-father and step-sibling might have contributed to Fluttershy's withdrawn personality and her love of animals (since she seldom met her real father and real brother since the divorce). Not to mention Fluttershy's childhood acquaintance, if not friendship, with Rainbow Dash.
Finally, thanks largely to "Shy's" unstable family life, she and "Dash" when they grow up finally move to Ponyville.
If Fluttershy's step-sibling was male, and she had a stormy relationship with her step-brother and step-father, that might have given her a preference for association with fillies instead of stallions, and contributed to her acquaintance/friendship with Rainbow Dash, and later with Rarity and Pinkie Pie when she moved to Ponyville.