So my cousin told me about this book and I really want to read it but she doesn't remember the title.its about a girl who falls in love with a guy and he with her but he has to marry her older sister because in that time the daughters have to get married in order. It's a really sad story apparently where the two can't be with each other. Also, in that time I think its expected to have an arranged marriage

Is it A Tale of Two Sisters by Julia Quinn? (In the anthology, Where's My Hero?)

"Julia Quinn's story is my favorite out of the three and it's Ned Blydon's turn to fall in love. Problem is he is engaged to one sister, and he is feeling things for the other. Of course, he is upset about nothing because it turns out his fiancee has other plans that do not include Ned. Maybe Ned can have that love match that his sister Belle keeps prattling on about. Ms. Quinn's trademark humor, romance, and characterization is plenty evident in this novella."

Ill Google it and ask my because thanks