Complex sentence: He offers them "aqueous martinis", (water with a olive in it served in a fancy glass), and introduces them to his wife Esmé Squalor, who is concerned about what's "in" and the city's sixth most important financial adviser.
Split into simpler sentences: He offers them "aqueous martinis", which is simply water with a olive in it served in a fancy glass, and introduces them to his wife Esmé Squalor. She is a very "in" person and the city's sixth most important financial adviser.