Complex sentence: He became the rabbinical leader of Toldos Aharon movement of Hasidim, based in the Jerusalem neighborhood of Meah Shearim; when Toldos Aharon's previous rebbe died, he joined the split-off named Toldos Avraham Yitzchok.
Split into simpler sentences: He became the rabbinical leader of the Toldos Aharon Hasidic movement, based in the Jerusalem neighborhood of Meah Shearim. When Toldos Aharon's previous "rebbe" died, he joined the split-off group named Toldos Avraham Yitzchok.