Complex sentence: The three are reluctant at first, what with Clary being Jace's sister, but after Izzy insists that she too would kiss Alec if only to free him, Jace kisses Clary.
Split into simpler sentences: The three are reluctant at first, what with Clary being Jace's sister. But after Izzy insists that she too would kiss Alec if only to free him, Jace kisses Clary.