The reason I believe, if not mistaken, that the brothers kept hiding things from Hari is due to the societal norm that women shouldn't "know too much beyond their expertise". Women in the nobility were expected to "keep quiet and live like a princess", and I believe that the brothers wanted her to live a carefree life that women "appears to" or should have during that era. However, the author wanted to change this imposed role/image and prejudice on women throughout the story, that is why we can sympathize with Hari about her brothers' unjust decision to keep hiding things from her. We will have to wait and see. I hope this makes sense for those that are wondering!