Complex sentence: Her mother would take her to visit Kanin's grandmother who lived in the Bronx, and it was there that she became devoted to the theater when she saw a matinee of "Idiot's Delight" starring Lunt and Fontaine.
Split into simpler sentences: Her mother would take her to visit Kanin's grandmother, who lived in the Bronx. While there she became devoted to the theater when she saw a matinee of "Idiot's Delight" starring Lunt and Fontaine.