She's always felt inferior to Sara.
Who wouldn't? She's prettier, smarter, a better swimmer, and is always the one who gets favoritism from their parents and the rest of their family.
"Sara is just so talented"
"Oh Sara, you just get prettier every time we come visit"
"Sara won another trophy? If she wins any more of those she won't have any room for them"
And she's expected to be just like her.
"Emily, do your stroke like Sara does hers."
"Emily, why can't you get straight A's like Sara does?"
"Sara is a machine, Emily you should try to be more like her"
She understands that Sara has a lot of pressure put on her, particularly from their father.
"If you don't come to the meet tomorrow with your absolute focus, you will embarrass our entire family"
"Junior Nationals are coming up and I expect you to qualify in every stroke"
"You represent this family, and we don't slack off"
Sara can make her feel like crap without knowing or meaning to
"Guess what Em? I just broke three school records in one meet"
"I made the honor roll the fourth year in a row"
"No Emily, you have to dive like this, keep your head to the side, your arms straight, and remember to watch the flags"
And the worst part is that even though she wants to hate Sara, she just can't. Sara can't help being perfect. But she can hate how she feels around her. Because just being compared to Sara "The Machine" Kessler, even in positive way makes her feel
Worthless
Shameful
Ugly
Stupid
Pathetic
She can't help what she is. She was not, is not, and never will be the machine everyone expects her to be. She's just the second best sister, the screw up compared to everyone's favorite. She's just Emily Kessler.
The HUMAN….
….And it kills her.