Ok here another idea that I think makes sense and is logical. Princess Cadence is Celestia's niece and Blood blood is her nephew. If this is the case, the two are either siblings  or cousins. Does Celestia have a sibling we don't knkw about? Luna is the only known sibling, is she their mother? 
Prince Blueblood ID S1E03

Possibly the sibling or cousin of Cadence

Princess Cadance Crystal Pony ID S6E2

Possibly Blueblood sibling or cousin