(It seems that it took shorter than I expected to finish this drawing – a few hours instead of a few days, if I can keep on focusing.)
 

 
Sunset: “More, more, MOOOORE!!! I want to see them all burn! Ahahahahahaha!”
 
Sunburst: “Chill, sis. You’re scaring everyone with your addiction to setting stuff on fire. Can you please do something else instead?”
 
Here is a picture of how I sometimes imagine Sunset and Sunburst to be, as fire or sun ponies (a bit like Celestia or Daybreaker) with the ability to control fire.
 
Sunset is the crazy older sister who likes to burn things, while Sunburst is the no-nonsense little brother who is annoyed by his sister’s love of destruction.
 

 
(Seriously, this is a nice picture. What is it with the downvotes?)