Thematic Bible: Not to defile themselves for the dead except the nearest of kin

The Lord said to Moses: "Speak to Aaron's sons, the priests, and tell them: A priest is not to make himself ceremonially unclean for a [dead] person among his relatives, except for his immediate family: his mother, father, son, daughter, or brother. He may make himself unclean for his young unmarried sister in his immediate family. read more.
He is not to make himself unclean for those related to him by marriage and so defile himself. "Priests may not make bald spots on their heads, shave the edge of their beards, or make gashes on their bodies. They are to be holy to their God and not profane the name of their God, because they present the fire offerings to the Lord, the food of their God. They must be holy.