Blue Moon is the son of Rarity and Fancy Pants. They had a divorce when Fancy Pants found out Rarity had been cheating on him.
 
Emerald and Jewel are the daughters of Rarity and Spike. Spike knew Rarity had a husband, but he never knew that they were still together.  
He was actually doubting whether marrying her was a good idea since she had cheated once. She might do it again.
 
Jewel is born with dragon wings, having more pony features while her sister, Emerald, possess more dragon features.
 
Jewel and Emerald liked making fun of Blue since he doesn’t have a mother. But Blue always reminds them that their mother was the one who ruined his father’s life. That seemed to always trigger the twins, making them yell at him that he was wrong, that their mother would never cheat on their father.
 
“You’re right, your mother never cheated on your father. But she did cheated on my father with yours.”
 
That seemed to put the twins in their place. They might seem to have shut their mouths then. But they always make fun of him the same way.
 
“Your father was the one who cheated!” Jewel yelled.
 
“That’s where you’re wrong. My father worked his horn off to earn bits ever since he moved to Ponyville. But our mother has been cheating every time when she got off work.” Blue Moon knew this because he always waited for his father to return in an empty home every at night.
 
Rarity was usually seen with Spike when she was still together with Fancy Pants.
 
“If you’re going to accuse my father of cheating, bring me proof. I have witnesses to prove that our mother cheated on my father.”
 
“Rarity isn’t your mother!” Emerald and Jewel both exclaimed.
 
“No, not anymore. I don’t have a mother who isn’t loyal to her own partner.” with that, he walked away, leaving the twins to calm themselves of their anger.
 
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