You Can't Quit

The girls have been taking piano lessons since the beginning of the year.  At first both girls learned at about the same pace.  They shared songs and learned the music notes together.  Then, the blond twin began to pull away.  A few lessons ago, their piano teacher looked at me and said, "She gets it."
Since then the blond twin has been speeding past her sister.  She plays with joy and bounces to the songs.  She reads the music and teaches herself new songs to surprise their teacher.  Sometimes she teaches her sister the songs.  She just gets it.
The brunette twin is moving along at the pace you'd expect a first-time, seven-year-old to learn.  She's learning at a good pace.  She's learning new notes.  She's perfecting songs.  She's just not doing it as quickly as her twin.
It upsets her to keep a song for a week longer than her twin.  She cried one day when she thought she played a song perfectly and her piano teacher corrected something.  She takes being at the same level as her sister very seriously.  It ruins her day every time she leaves lessons without getting a new song.
One day she said to me, "I don't want to take piano lessons any more."  I said, "Not an option.  Just because your sister is moving along more quickly doesn't mean you're going to quit.  You're going to keep playing until you get really good at it.  Then you can decide whether or not to quit."
Despite the fact that she complains about lessons, she glows when she's with the teacher. She likes to learn new songs and delights in her sister's success. She said she's enjoying the lessons.
Now, if she was really, really bad at piano, we might have let her quit.  Some people just don't have any musical talent.  This is not her problem.  She's a good piano player, but it eats at her that it all comes so much easier for her twin. 
The real problem is the precedent it sets if we let her quit.  There are going to be a lot of things that one of them will do better than the other.  This doesn't mean the one who has to work harder gets to quit just because it takes more effort.  They both need to learn that being good at something takes a lot of practice and effort.