Okay, this opens up a huge topic and honestly we are all guilty of it..... so here is my suggestion based off experience. Stop compairing your children. Remember we are all born individuals and it doesn't/shouldn't matter which child does what better/more often/worse, etc. etc. They are their own person with their own needs.
I did the exact same thing. My son was born very bright, no problems at all catching on with anything. My daughter on the other hand has a severe speech and language delay and has to be in Special Ed Classes at school because she simply isn't as fast. I reconized the issue right away and at first I did the whole....well, my son isn't like this so why is she questioning..... put all that to the side and just consintrate on your daughter being your daughter and her needs.
I took my daughters concerns to the school nurse and she then sent the concern to the school counceler.... There are many tests that the school can do to see if she may be suffering from a learning disability that may be affecting her confidence in school. Her age would say that's exactly whats going on anyways. So, I would go to the school and request that she be tested. My daugter is in Reading/Launguage and Speech help and has earned her confidence back and is doing great in school now. It was the best choice I ever made. But remember, the first choice is to "not compare". If she ever hears you with school officials or even on the phone with another person compairing her to her siblings, than that's what she is retaining.... not the concern, but the compairing..... Hope that makes sense......
Oh, by the way..... Kudos to you for catching tis.... It will change her life as soon as the diagnoises is made...... Go back for that second opinion (demand it from the school) and don't skip a beat doing it...... Your child may never know how hard you faught for her, but your own heart will....