They are Lullaby and Symphony, they are two musician ponies.  
Lullaby is a calm and elegant pony, she always prefers to wear a dress or something fancy.  
Symphony is more daring and rebellious and she doesn’t wear almost anything, neither on special occasions and for the other sister this is not a problem.  
They have:  
3 forelegs, 2 hind feet, 4 lungs, 2 hearts, 2 digestive systems that converge.  
Lullaby is the singer of the duo, she is the dominant twin and she controlling both legs, but she has only one arm.  
Symphony is the musician of the duo and she controls the remaining two arms, which she uses to play the guitar or other instruments.  
They became orphaned at 5 years. When they fled from the orphanage they wandered from house to house to look for a cozy home and someone to love her.  
At the age of 7 years they have been adopted by a married couple with a daughter.  
The husband is a scientist who makes genetic experiments, the wife is a mutated pony with pony-bat ears, 4 wings and she has a griffon tale and their daughter has 6 legs.  
Lullaby and Symphony have found a family in which they feel “normal”.