Complex sentence: Katie's best friend is her older sister Lynn, whom Katie looks up to as the most intelligent person she knows, citing Lynn's ability to beat their Uncle Katsuhisa, a self-proclaimed chess grand master, at his own game as an example.
Split into simpler sentences: Katie's best friend is her older sister Lynn, whom Katie looks up to as the most intelligent person she knows. She cites Lynn's ability to beat their Uncle Katsuhisa, a self-proclaimed chess grand master, at his own game as an example.