@Splinter17  
Cousins because it’s funny. Ignore book canon on this.

Cadence was born a pegasus. She became an alicorn when she used a magical amulet to defeat some evil so and so. (I forget the details). She was probably adopted by Celestia as a niece.  
Blueblood is probably the son of an advisor and Celestia affectionately calls him nephew. I don’t think they’re related, but they each have the title of niece/nephew.

@HJSDGCE  

I always had the idea of Cadance being Blueblood’s cousin… And then I had another idea of them fighting and arguing which then ends with Mortal Kombat!

Seeing as how non-alicorns can do things like raise the sun/moon and travel through time, I hesitate to call the sisters goddesses.

Gods? No. Powerful and long-lived. but not gods.

From what we can tell, Luna and Celestia have a line of mortal unicorns nieces and nephews, the mortal royalty, so its assumed that Blueblood is closely related to Cadance.
 
They made it established that Blueblood was in fact related to Celestia, so therefore Cadence is as well.
 
This unicorn dominance suggests that Celestia and Luna were in fact daughters of Platinum, but there was a third mortal sibling, most likely a brother, that had children and passed on. Somehow, who-ever the father is, Luna and Celestia became gods, and all female descendants of the mortal royalty become alicorns, but not goddesses.

@Splinter17
 
I assumed Celestia and Luna have adopted siblings/siblings-in-law.

How did she know it was Blueblood if he’s wearing a mask?

You know, I have to ask… are Cadance and Blueblood in any way related? I mean, they’re supposed to be the niece and great grand nephew of Celestia and Luna respectively. Come to think of it, how do Celestia and Luna have a niece and great grand nephew if they apparently do not have a third sister or brother? Sure would be nice of the writers filled in this plot hole…

I like the masquerade cutie marks.