During periods of mourning, the high priest was not allowed to leave the sanctuary compound. This was done so that no impurity might defile the high priest, hindering his service in the Tabernacle. Leviticus 21:12 does not mean that the high priest lived in the sanctuary, only that his duties there took precedence over family ties, even when his parents died.
12 He shall not go out of the sanctuary, lest he profane the sanctuary of his God, for the consecration of the anointing oil of his God is on him: I am the LORD.