Return to Mariposa by Anne Stuart

Kitty Whitehead has never gotten over her banishment from Mariposa, the family home in Spain. When her glamorous cousin Bella comes up with the daring suggestion that they take each other’s place, she reluctantly agrees, for the chance to say goodbye to the house and her estranged family.
But turning mousy Kitty into the glorious Bella is harder than she thinks, even if everyone seems to be fooled. Everyone but her hateful almost-cousin Ian, who’s getting too close to the truth and too close to Kitty, as he invades her dreams and makes her question everything she’s ever known to be true.
Someone is out to kill Kitty…or Bella. Who wants her dead, and why? Could Bella’s old enemy turned lover be the greatest danger she’s ever faced?
A modern story of romantic suspense and masquerade, where no one is as they seem.