Complex sentence: In 1916, she joined her older sisters, Cornelia and Katherine, in Madison Wisconsin, where both were Cornelia as in Graduate School and Katharine was teaching, during which time she attended the classes at University of Wisconsin.
Split into simpler sentences: In 1916, she joined her older sisters Cornelia and Katherine in Madison, Wisconsin, where Cornelia was in graduate school and Katharine was teaching. During which time she attended the classes at University of Wisconsin.