Complex sentence: Later, when her mother became ill, her aunt Madge Spear (whom Stevie called "The Lion Aunt") came to live with them, raised Stevie and her elder sister Molly and became the most important person in Stevie's life.
Split into simpler sentences: Later, when her mother became ill, her aunt Madge (whom Stevie called "The Lion Aunt") came to live with them. It was Madge Spear who raised Stevie and her older sister Molly, and who became the most important person in Stevie's life.