The answer is nephew. If we look up this word in a dictionary, we will find that "nephew" is, in fact, the son of your sister or brother, or the son of the sister or brother of your husband or wife. Nephew is the term used for boys; but if we want to refer to the female counterpart of nephew, we should say niece, that is, the daughter of your sister or brother, or the daughter of the sister or brother of your husband or wife.