A "live-in son-in-law" is the best I can do. I don't believe there is one word that covers that specific case in English. A derogatory term would be "mooch", but that is a general term that applies to anyone who takes freely from others without intending to repay or otherwise compensate the giver. It has only negative connotations, whereas the phrase I made up for you is neutral, to my ear anyway. Maybe someone else will have a better idea for you! :0)