SOLVED. YA book before 2013 about a girl who lives on a ranch and has to fill in for her sister and learns how to wear dresses and have proper etiquette falls in love with a handsome rich guy. Spoilers ahead. [s]

[Noel]

This is a ya book I read before 2013 possibly with a hot pink cover and maybe the girls name is Darcy. She lives on a horse ranch and has to leave to fill in for her sister at a maybe fairy godmother type service where they teach her to be a “lady” with dress and etiquette to go I think undercover. She meets to handsome men who one is blonde and SPOILER turns out to be an evil (i think spy) and the other SPOILER she ends up marrying after he rides in on a horse to her house. The sister who the main character thinks is flaky turns out to be doing (I think) spy work or is engaged. But I know the sister crashes a boat. This is a dirty book with erotic scenes one takes place in a public dressing room.

[Mai]

The Cinderella Rules by Donna Kauffman? I doubt this is a young adult novel, though... specially if there are erotic scenes!
Her name is Darby so you were close! :-)

[Noel]

Yes that is it thank you!