Answer
Mordecai Kaplan's son in law is Ira Eisenstein. Mordecai Kaplan was a rabbi and Jewish philosopher who founded the Reconstructionist Judaism movement. Ira Eisenstein was a rabbi and Reconstructionist theologian who was the son-in-law of Mordecai Kaplan. Eisenstein was a prominent figure in the Reconstructionist movement and was the editor of the movement's journal, Reconstructionist. He also served as the president of the Reconstructionist Rabbinical College from 1963 to 1977.