Complex sentence: His mother, Helen Callaghan St. Aubin, played for the All-American Girls Professional Baseball League, which was depicted in the movie "A League of Their Own", making them the only mother/son combination to have both played professional baseball.
Split into simpler sentences: His mother, Helen Callaghan St. Aubin, played for the All-American Girls Professional Baseball League, which was depicted in the movie "A League of Their Own". Casey and his mother represent the only mother/son combination to have both played professional baseball.