Let's admit it, this is RD's brother, one of the many "Rainbow _ash" siblings of hers. They're all the same except their cutie mark (which always has a cloud), fur color, and first letter of second name. We got Rainbow Smash... he's green. Rainbow Cash, she's gold. Rainbow Rash, who's red. Rainbow Trash, who's orange. Rainbow Splash, who's white. Rainbow Flash... well, the list goes on.
Just consider how much they like brothers or siblings in general compared to parents. Twilight's parents never got much screen time and appear most often in crowds at big events for either Shining Armor or Twilight. Pinkie's parents only showed up in a flash back. Rarity's parents only appeared to dump Sweetie Belle at her house and leave. We have seen practically every Applejack relative except her parents. Big Mac and Shining Armor have become memorable characters. Even Maud Pie got more recognition and screen time than any of the parents. Considering all that and the fact that this guy looks young, he would have to be Rainbow's brother.
Well I'm not sure any of that is intentional indication. It's interesting though how out-of-town family isn't even mentioned unless relevant. Is Fluttershy an orphan?! Anyway, my reasoning for brother is because he just looks so darn much like her. If he were RD's dad, his wife would have to be blue and... I don't even know mane-wise. I guess another rainbow mane? My point is, as far as genetics go, it's far more likely for siblings to look so similar than it is for parents and children. It would be like if Twilight's mom had Twilight's mane, instead of Twilight's mane merely being a combination of her mother's purple-white and her father's navy blue. RD would have to be almost a carbon-copy of her father if this were her father! On the other hand, siblings can look identical even when they're not.
She is actually just a bg pony that appears often, and is most likely unrelated, and she has interacted with rainbow dash before, without any actual acknowledgement, as well as her voice being to young sounding.