Hastings' Dictionary of the Bible (1898)
SIS'TER'S SON. " In biblical usage, this word is not limited to our sense, but, like "mother," "father," "son," "daughter," has a far wider application. Thus, in 2 Sam 13:2, it means a step- or half-sister, in Matt 13:56 a cousin, and in Rom 16:1 merely a fellow-believer.