Complex sentence: Vivekananda held her in great esteem and would write to her, not only on mundane issues but also about his inner feelings, an aspect which he did not share with any body else either the brother monks or his "Madras Boys".
Split into simpler sentences: Vivekananda held her in great esteem and would write to her on both mundane issues and on his inner feelings. He did not share his inner feelings with anyone else, including the brother monks or his "Madras Boys".