Half Brother by Kenneth Oppel, Scholastic and similar items

For thirteen years, Ben Tomlin was an only child. But all that changes when his mother brings home Zan--an eight-day-old chimpanzee. Ben's father, a renowned behavioral scientist, has uprooted the family to pursue his latest research project: a high-profile experiment to determine whether chimps can acquire advanced language skills. Ben's parents tell him to treat Zan like a little brother. Ben reluctantly agrees. At least now he's not the only one his father's going to scrutinize. It isn't long before Ben is Zan's favorite, and Ben starts to see Zan as more than just an experiment. His father disagrees. Soon Ben is forced to make a critical choice between what he is told to believe and what he knows to be true--between obeying his father or protecting his brother from an unimaginable fate.