Their sibling dynamic is cuter.

Probably because a lot of your family’s counterparts look down on incest?

@Zerowinger  
“Incest is the best. Put a royal to the test.” -Old French Proverb^{citation needed}

@Zerowinger  
Royal family incest isn’t as stereotypical as redneck incest?

Because it’s good not to promote stereotypes.

@cold
 
Changeling queens aren’t spoiled for choice; if they’re not on infiltration and want some cock, they only have their sons to choose from.

@Brass Beau  
Well, insect is an anagram for incest.

Does it count as incest when a changeling is involved?

Aristocratic incest > Peasant incest

@landshark15
 
That ep gave me the headcanon that Apple Bloom is their daughter. No other breeding age Apples, their parents are dead, yet no AB…

I’d thought the episode, Where the Apples lie would have at least encourage people to make more porn of AJ and Big Mac considering the sibling rivalry they had in that episode.

@Kalenz  
Probably no one thought of creating a catch-all tag for Sparkle family pairings. Applecest has no aliases or implied tags attached, incest tag excepted, and just kind of came about organically.

@Kamazeustra  
Then why is there no sparklecest tag? :/

@Kalenz
 
All Apple combinations are contained in the Applecest tag. Sparklecest is only an alias of the Shiningsparkle tag. If the other related tags were included, Sparklecest would probably be the largest.

@Background Pony #B9A2
 
Additional:
 
ApMac+:
 
Apple Bloom is likely their child  
It gets lonely on a farm
 
ApMac-:
 
Mac is a married stallion  
If she gets pregnant, it’s either abort or the secret gets out
 
TwiShi+:
 
Alicorn pussy is best pussy  
Gotta keep that bloodline pure  
They get a ton of time together  
Royal threesomes with Cadance, at the very least  
Shining has a kid already, so he’s potent for sure
 
TwiShi-:
 
AJ definitely has way better fuck game with muscle control and endurance  
Cadance may not take well to her husband cuckqueaning her

Let’s legit try to break this down real quick
 
Applemac-
 
Pros:  
country stereotypes  
no cannon AJ ships (until the very end)  
proximity  
the name “big” macintosh
 
Cons:  
a LOT of cannon/supported Big Mac ships  
few sibling focused episodes  
Big Mac can’t flirt
 
Shiningsparkle-
 
Pros:  
BBBFF (the song AND the title)  
Twilight acts possessive when he’s getting married  
Cadence might be into it, Twi was a bookish shut in  
everyone hates FlashLight  
Shining’s a dreamboat  
Friendship is Witchcraft: Foaly Matripony
 
Con:  
infidelity  
distance  
Twi has A LOT of supported ships (even if none of them are near cannon, except Flash)
 
In conclusion, I think it’s clear that Shiningsparkle has a lot more going for it.

Because you are low class mudpony trash and she is high class superior unicorn.

Because Hillbilly incest is pretty mainstream, high society incest is more taboo and more appealing.

That would also make Twilight more incestuous than you, Applejack. You had better step up your game.

Twilight: “Simple Applejack; It’s not the size that counts, its how you use it.”  

@Kalenz  

 
Horrible, but likely

WHAT WHAT THE FUCK? HOW IS THIS CONSIDERED SAFE?

Umm.  
Applecest is pretty much the icon of mlp incest.  
twicest (986)  
applecest (1833)  
AJ’s problem is that she has to share with Applebloom a bit ;)

Is that really a question that you should be asking AJ?