You have really captured Simon's feelings for River extremely well, HopefulNebula! His devotion to his sister, regardless of her supposed appearance as a simpleton, bled through his recollection of their shared childhood, though having her first spoken word be such an advanced mathematical concept is fitting.
Does make you wonder...if this is close to where the writers of Firefly would have taken River's history, questions can be raised about whether River should have kept her mouth shut and avoided being singled out as a super-genius. Then again, who knows what the elder Tams would have done if they though River was mentally handicapped and didn't want to deal with her :S