@Starswirl  

 
>but i can change to anything you desire my little pony…
 
>ends in sexual results

>let`s have sex

@Starswirl  
I’m saying the only two options which are shown in the show, as they haven’t stated the two have any other siblings.
 
Faust however has stated the Luna and Celestia had no siblings or children, nor others to outrank them. Blueblood is their nephew from their mother’s side, roughly 52 times removed.
 
But since Faust is no longer on the team, I’m not sure if that should be considered the answer.

@VermiIIion  
“canonical option would be Luna had a child with someone”  
Not quite. They could also have had a third sister or a brother who founded the line. Could have been an Alicorn, could have been a regular pony.

@Midday Shine
 
All we know is that there is Celestia and Luna. Since Blueblood is Celestia’s nephew, the only canonical option would be Luna had a child with someone, who had a child, who had a child, etc. Celestia does have a cousin who is King of the Sea Ponies, but I don’t know if they’re just adopted family or not.
 
Besides, Celestia did say they have never seen the birth of another alicorn, so it’s confirmed that the two sisters themselves never gave birth to another alicorn or had relatives who were alicorns.
 
 
I’m pretty sure Twilight just happened to win the genetic lottery. Not every single powerful unicorn is related to the two sisters. Twilight is already banking on Mary-Sue levels here, considering she’s:
 
An alicorn  
Incredibly powerful even without being an alicorn  
Incredibly intelligent  
Student of the royalty  
Related to a member of the royal guard  
Stepsister to an alicorn
 
Making her distantly related to Celestia would just be the icing on that cake. Twilight is only saved from that title due to her personality.
 
And it was said that the two became alicorns because of their actions, not their power. In the comics, Trixie became an alicorn in an alternate universe because of her Humility. Twilight is Friendship which also happens to be Magic. Cadance is Love.

@VermiIIion  
Oh, I do know about the adoption thing. Still, if Blueblood somehow managed to be Celestia and Luna’s distant relative (and, by extension, have a tiny percentage of alicorn genes), then how can we be sure Cadance’s and/or Shining’s case is not similar? After all, we don’t know how numerous alicorns were back when Celestia and Luna were foals (or before they were born).  
Also, an alicorn ancestor (not necessarily common, and not necessarily one of the Royal Sisters) would go a long way towards explaining how it was possible for one pegasus and one unicorn to become alicorns; not to mention Twilight Sparkle’s almost ridiculous amount of raw power. ;)

Flurry looks like a swan

@Midday Shine  
Cadance is only Celestia’s adopted niece. And I sincerely doubt any blood relation to Celestia outside of Blueblood which is very, very distant.

@VermiIIion  
Or Cadance (or maybe Shining?) is actually somehow related to Celestia. You know, a great-great-great-…-great-child or a cousin many times removed. :P

@VermiIIion
 
Heyooooo~!

@Dirty Bit  
I’m not sure that’s applicable when Flurry is an alicorn.
 
Or maybe Flurry’s a grave robber? ;)

@Crystal Neighybánya
 
Wouldn’t it be cradle robbing? :U

This is going to end with Chrysalis seducing Flurry Heart, isn’t it?

Flurry heart was kidnapped and become another nightmare moon

@Starswirl
 
“Oh, and just as catty~!”

Nah, she looks more like Celestia.  
Conspiracy?!

“And you look like something I accidentally stepped on yesterday.”