Leviticus 21:3 "For her, he may become impure"

The Torah regards priests as very holy.

They should not become impure (see Negative Mitzvah 166).

Therefore, they must not come in contact with a dead body (see Positive Mitzvah 107).

Nevertheless, the Torah commanded them to attend the funeral of their closest relatives.

The Torah lists these relatives as follows:

Mother; Father; Son; Daughter; Brother; and unmarried Sister.