We need to see (or at least hear about) canon relations to RD and Fluttershy, even just to say they're estranged or dead in the present. I'd even be happy if they were antagonists (or snared in the plans of the antagonists) somehow.
Also could be an uncle. Or a cousin. My personal favorite is that he's her boyfriend. But I think brother makes the most sense. And all her other siblings have rainbow manes with different pets. There's Rainbow Smash, Rainbow Blast, Rainbow Clash, Rainbow Rash, Rainbow Slash, Rainbow Cash, Rainbow Mash, Rainbow Bash. You know, to name a few.
Now this is interesting. Here we see that Rainbow Dash is still a kid, while the other ponies we've seen have been high school aged. I can understand that for Big Mac, Shining Armor, Cadance, and Bulk Biceps, but what about Vinyl Scratch and Octavia? How much older than the main six are they supposed to be? I'm asking this because of how young Rainbow appears here. She's obviously not a teenager.