Complex sentence: There he met the physician and health food promoter John Harvey Kellogg, co-inventor of corn flakes breakfast cereal, who became his mentor, and married Kellogg's niece, Lena Celestia Kellogg, in 1897.
Split into simpler sentences: There he met the physician and health food promoter John Harvey Kellogg, co-inventor of corn flakes breakfast cereal, who became his mentor. He married Kellogg's niece, Lena Celestia Kellogg, in 1897.