It all started with Charlotte. Originally it was a romance novel, where Charlotte was living much poorer than she actually is-- that plotline hasn't actually changed, though her reasons for it have-- and fell in love with Daniel's precursor, who flipped his shit when he found out she was rich, typical romance novel misunderstandings, etc. Then Miranda (originally named Siobhan) and Jack (originally named Ryan, and it may still be his middle name) showed up, Miranda via getting stabbed, and the story built from there.

Interesting fact: the Hennessys were Irish Catholic until I realized that I could not write a story about classism in America without also addressing racism. Hence their Hispanic origins. Still Catholic though (nominally).