Leviticus 21:1-4

The LORD said to Moses, ‘Speak to the priests, the sons of Aaron, and say to them: “A priest must not make himself ceremonially unclean for any of his people who die, except for a close relative, such as his mother or father, his son or daughter, his brother, or an unmarried sister who is dependent on him since she has no husband – for her he may make himself unclean. He must not make himself unclean for people related to him by marriage, and so defile himself.