Complex sentence: Born in County Dublin, Stanislaus was considered a "whetstone" by his more famous brother, who shared his ideas and his books with him, he was three years younger than James, and a constant boyhood companion.
Split into simpler sentences: Born in County Dublin, Stanislaus was considered a "whetstone" by his more famous brother, who shared his ideas and his books with him. He was three years younger than James, and a constant boyhood companion.