While it is legal to marry a first cousin, I don't think it's ok to sleep with them as a habit, and definitely not ok to sleep in the same bed as an aunt. If she needs to stay overnight and there is no sofa, one of them should sleep on the floor. You will have no way of knowing whether this will happen while you're away, but the fact that he thinks it's ok to sleep with multiple cousins would make me feel rather queasy about the whole situation.