I think uncle and niece fits better for them given Spike and Twilight’s relationship.

Spike fits many roles

@Background Pony #D437  
Yea, Spike’s obviously something like father/mother figure to Twilight. …What? He’s making her breakfasts, he’s telling her to go to sleep when she’s too buried in her studies, calms her down when she’s freaking out, etc.

@CogWeaver  
It doesn’t really make since though to just go off someones else’s personal perspective, especially here when there’s clearly a contradiction from the authors idea. And I’m pretty sure there’s a precedent for this already, as Tsispike pics aren’t usually tagged as incest.

@MadVillan  
Word of Faust has always been iffy for many given that we haven’t had any specified explanations on how far in advance she helped plan out the show - it seems like she had influence in the way things ended up going beyond S2, but how far is unknown. The comic thing is good to know, though, at least until/as long as the show doesn’t contradict what it says. It does fit with Spike’s offhand comment of “You’re not mommy” and a sort of underlying premise to Dragon Quest.
 
So really when it comes absolutely down to it, he would more likely be an uncle figure to Flurry, since he’s more comparable to a sibling to each of the mane 6 and especially Twi.
 
All that said, though, the pic does refer to them as being cousins, so both tags still apply (even if on a technicality)

@CogWeaver  
I don’t hear that brought up a lot, the last times this topic was brought up it was when Lauren Faust stated Celestia raised Spike. Then recently with the new comic Spike being Twilight’s first real friend.
 
And really at this point all the mane-6 are family, so the brother sister thing kinda goes for all of them. I’m all for adding joke tags but this situation here is pretty funny since they contradict each other.

@MadVillan  
Well, there’s an offhanded comment he makes and then sort of an underlying concept for a full episode that Spike doesn’t really consider Twilight to be a full mother-figure for him. But on the other hand, it’s more or less assumed that she did raise him from a hatchling (unless, on another hand, Celestia took on that role while Twilight was performing her studies)
 
And then there’s the factor of his mentality and maturity, how he pretty much acts a near-equivalent age with the mane 6 and takes on a “little brother” role with Twi, which I think is the artist’s thinking here with calling the pic “Surrogate Cousins”.
 
Really, though, I think it’s good to have both tags so people can think what they want about Spike’s varied relations with other pones.

@Itsthinking  
Wouldn’t it be none then? Unless there’s an episode or something I missed that states otherwise. Also, are Shining and Spike pics tagged Uncle and nephew or brothers?

@MadVillan  
No, it’s just that Spike has an unspecified roll in Twilight’s family even though it’s clearly parent and child, but others prefer adoptive brother.

@MadVillan  
It really depends what his relationship is I guess.

>Uncle and niece tag  
>Cousins tag
 
Hillbillys confirmed?

Dawwwwwwww