I wonder, does Norn still hold a grudge against her brother for what he did to Paul ? That was quite some time ago though.
I doubt that plays any concrete role anymore. It has been an excuse for her to not get close to Rudy, but isn't her anguish due to a lack of self-confidence and an abundance of self-pity? Aisha is a genius, no matter how you look at it, and she has been tormenting the not-genius Norn since the beginning. Rudeus isn't a genuine genius, per se, but he's as much of an apparent genius as one could be. He was never a real child in that world, thus mastering everything very young, plus, by coincidence, he has a godly mana pool, silent magic casting, and such. So, from Norn's point of view, the big brother is another unreachable genius. I don't believe Norn's problems are any more or any less than this, though I suppose the next chapters will tell the truth.