HYUK these two look like Elphaba and Glinda from Wicked!
 
Okay, so I know I draw a lot of strained sister relationships. Well…I have like 3 all together but this one is the worse.  
Thistle Trill and her sister Utopia Swansong are about a year apart with Utopia being the younger one.
 
Utopia is a psychopath. She’s not aggressive and she doesn’t check all the characteristics but she is a psychopath. She’s been taught empathy but has no regard for anyone’s wellbeing, and is extremely manipulative. She’s also a bit of a narcissist. Starlight and Treehugger work with her a lot. However, some of Utopia’s traits slip through the cracks. Most ponies know there’s something off about Utopia but she manages to be top dog at school. She gets amazing grades but she likes to torment her classmates if they don’t give her what she wants.
 
It’s always the pink ones…
 
The sisters have a terrible relationship as Utopia is the cruelest to her elder sister Thistle. Utopia knows that Thistle despises the fact that she was born an Earth Pony and she exploits that. She constantly belittles her older sister and questions her intelligence.  
As I said, Treehugger and Starlight put a lot of energy into having the kids get along. However not only is Utopia malicious, she also has a high IQ. She’s smart enough to see that her parents are trying to “make her better” and she plays along and puts on the facade of getting better.
 
She is not.
 
Thistle Trill tries to be better and prove her intelligence. However, she’s awkward and unfortunately…not quite as intelligent as she wishes she was.
 
These two are always trying to one-up each other and regularly have little spats. ONE way that Thistle is superior is in physical strength. Utopia only had to learn that once before trying new tactics that wouldn’t lead to her ass getting beat. Still, from time-to-time, she gets a swift hoof to the snoot. Bitch ain’t invincible!
 
EDIT: I think people may have questions about Utopia’s diagnosis. I haven’t fully explored the extent of her issues but I plan to shortly. I’ve based her character off someone I know personally and I did a bit of reading on the topic but I of course do not claim to be an expert. I’m always open to discussion.