“And they shall come into contact with no dead person to defile themselves. But for father, or for mother, or for son, or for daughter, for brother or for sister who has no husband they may defile themselves. And after he is cleansed they will reckon to him seven days. And in the day that he goes into the sanctuary he will offer his sin offering, says the Lord Yahweh.”
Contact with the dead always defiled and required seven days of cleansing (Numbers 19:11-13 compare Numbers 5:2; Numbers 31:19). (There are actually good medical reasons behind such a restriction, lest any disease has been contracted from the dead body). The priests were only to become involved with the dead in the case of close relatives, where there was no one else to take on the duties. Then they must got through the recognised period of cleansing. The sin offering was required because death was so closely related to sin.