What's it called?

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I went to the library today and rented the movied, "The other Boleyn girl" which is based on a book about two sisters, who are forced, by their uncle and father to seduce the King, marry him and have his child in order to make money since the family is poor. It's a really good movie and I wanted to re-watch it. Anyway, while I picked it up I remembered this other movie I watched while I was on vacation. It had almost the same plot and it was a really great movie, but I forget what it was called. It was about two sisters. There is an age difference, I think about 4-8 years. The younger sister falls in love with a guy her sisters age and fantasizes about him. At the same time, the sister likes him to, infact she sleeps with him and the little sister sees or finds out. I don't remember any more of the plot but I would be extremely greatful if someone could help me out with the name of this movie! Thanks.