@DragonBoi471  
Because that can only happen with new writers(sarcasm).  
Also there was a scene that was pretty much a recap of all the Rainbow Dash wonderbolt episodes and your say new writers completely ignore Canon events?  

Uncle.

Personal headcanon here:
 
I’m going with Uncle Rainbow Blaze. Her totally chill uncle who she likes to tell her stories to because he doesn’t go off the deep end with his praise. He totally knew she was a Wonderbolt and they went out for cider afterwards to celebrate.
 
He might love his brother, but he and his wife can be a bit exasperating after all.

Brother or cousin, they have to be some relation.

>big brother
 
that would be kinda sad… your parents only care about your little sister…

@PonyPon  
Or the youngest.

Saw a few spoiler and I still prefer Firefly and Rainbow Blaze to be Dashies parents. Stupid copyright crap (most likely the reason why it wasn’t Firefly)

if he was her brother it would kinda suck to live in a house that basically serves as a shrine to the only daughter. maybe he and bow hothoof are brothers? hell, i could buy twin brother. it would explain the differing cutie mark alongside the near-identical color schemes. he could totally be an avuncular mentor since bow hothoof doesn’t exactly have the figure of a great flier. maybe rainbow blaze takes better care of himself than his paunchy, saggy-looking brother?

Maybe uncle?

No way is he a brother.
 
Only an only child could get the kind of praise Rd got from her parents.

This is why I hate new writers: They completely ignore past canon events in the show in favor of making up their own, resulting in a ton of confusion and delay negativity within fandom.
 
It also means that everyone has to rework all of their headcanons and every fanart/fanfic/etc. that use them. :(

@Stotter  
Personally I’m not a fan of the brother option.
 
Cousins maybe.

Big Brother, case closed in my book.
 
Side note, how funny would it be if there was a one-sided ship between him and Fluttershy like with their siblings? Except it would be Fluttershy with the crush since fillyhood, but Blaze wouldn’t have been able to notice because she was equally shy around everypony, so now he’d be the one pony that wouldn’t see that she’s changed due to still being awkward around him.

Actually, Bow Hothoof has Rainbow Blaze’s colour scheme (almost) and had a similar mane style in the past. They probably are brothers, making R. Blaze Rainbow Dash’s uncle!

@melon frost  

 
He was a mentor. Doesn’t say he was her dad or anything.

@PonyPon  
No, but other than an animator’s input and a few design changes, they’re the same.

new dad best dad

Rainbow Blaze and Bow Hothoof are not the same. Rainbow Blaze appears in the background of Hearth’s Warmings Tale, and that’s a rather specific vector to reuse. So unless the showrunners say they’re the same character and explain Blaze in that ep, we should not consider them the same.

@Backgroundpony  
We barely saw the cutie mark the first time & it wouldn’t be the first time a cutie mark’s changed.

@Meanlucario  
Is it really that hard to take that he’s a different character?

@Backgroundpony  
magical uncle

@melon frost  
The episode definitely showed her as a only child.

@melon frost  
If we count the merchandise (that is often wrong with the names) then he has been named Rainbow Blaze.

or she has a brother lol