Leviticus 21:1-4

GOD spoke to Moses: “Speak to the priests, the sons of Aaron. Tell them, A priest must not ritually contaminate himself by touching the dead, except for close relatives: mother, father, son, daughter, brother, or an unmarried sister who is dependent on him since she has no husband; for these he may make himself ritually unclean, but he must not contaminate himself with the dead who are only related to him by marriage and thus profane himself.