Passionate Protection by Penny Jordan

Jessica Forbes will do anything to protect her cousin, Isabel, even pretend to be her—she just never imagined it would result in her agreeing to work with the captivatingly arrogant Sebastian Calvadores.
Sebastian seems to have nothing but contempt for innocent Jessica, but the callous aristocrat has her pulse racing whenever he enters a room…
She doesn’t know if she’ll ever be able to change his mind about her, but she’s finding it impossible to resist her longing for the Spaniard’s touch!

Originally published in 1983