[@Grey]
So far your description roughly aligns with what I had in mind as well.

I was thinking of making the little sister a prodigy, but one that lacked self-awareness and found it hard to believe she was one. 
If her older brother is well-accomplished, then she might grow up in his shadow and believe that hypercompetence is the norm rather than the exception, even if her own capabilities could outstrip her brother's. 
Maybe she admires him as well? A kind of hero worship? Which could also lead her into underestimating herself.

The matter of class is still up-in-the-air, but I was thinking of Tomes and Swords or Tomes and Knives/Shuriken/Whatev.