A Ring for Cinderella — Judy Christenberry

Susan Greenwood had expected a tip, not an offer of marriage!
One very run-down diner and three beautiful, independent sisters. Baby sister Susan is still getting used to the fact that she has two sisters she never knew about. Now, wealthy rancher Zach Lowery is offering her a life of comfort and security in exchange for being his wife. Only one thing's wrong here: he doesn't even pretend he's marrying her for love, he just needs a companion for his elderly grandfather. Susan's kind, generous nature leads her into taking him up on this offer. She never realized she would soon find her heart in jeopardy.