Complex sentence: Gittel descended from a long line of Hasidic Rabbis, most famous of whom was her grandfather, Rabbi Mordechai Leifer of Nadvorna, a Hasidic Grand Rabbi, of whom thousands flocked to for his blessings and miracles.
Split into simpler sentences: Gittel descended from a long line of Hasidic Rabbis, of whom most famous was her grandfather, Rabbi Mordechai Leifer of Nadvorna. A Hasidic Grand Rabbi, thousands flocked to him for his blessings and miracles.