Am I obligated to read the books my sister-in-law lent me?

Without my asking, my sister-in-law lent me three books (a series). I really don't feel like reading them right now. I'm reading other books, and I just don't feel like reading fantasy novels at the moment. Every time I p my bookshelf, however, I feel guilty when I see her books. She hasn't said anything yet. Am I obligated to just hurry up and read them so I can return them to her? Maybe I should buy the cliff notes.