Dorobou no Meijin

My name is Hatsuyuki. I'm almost 15 years old. "Do you remember Kawai-san from the used bookstore? I brought you there a long time ago. Become her sister." Megumi Kawai, the beautiful proprietor of a used book shop, wanted me, and so my sister gave me to her. A strange life begins, making sure that I am loved and she doesn't get bored of me. I must complete this task in order to protect my sister's life. What if I wasn't obligated to... Would I come to like her of my own accord? (Source: Shogakukan, translated)