Your relative is asking for trouble. If you live in the same house and they won't stop drinking, it's time for some sneaky action in watering the bottle down.
OK, well back in my early college years this was not uncommon, but I can tell you the consequenses.
Short term use will cause, nausea and weight gain..and were talking a LOT of calories in half a bottle) Long term problems can occur with your liver. Processing that much liquer every night is giving it quite the work out, its likely the liver would wear out long before its normal date!