Leviticus 21:3

and for his sister, a virgin who is nigh to him, and who hath neither been betrothed, nor married to a husband, for her he may defile himself.

who is near to him i.e., a full sister, with whom he shares both a father and a mother.

ולאחותו הבתולה, “and his sister as long as she is still a virgin;” The Talmud in tractate Nazir, folio 47, commenting on this verse, sees in the repetition of the word ולאחותו, “and on account of his sister,” an allusion to the contrast with a corpse who has no one claiming it, and which therefore even the High Priest is not only permitted but commanded to defile himself and bury, out of consideration for human dignity. We find the following comment on this subject in Maimonides, laws concerning mourning, chapter 3, halacha 8: if a High Priest encounters a corpse along the way, and he finds himself so far from the nearest human being that even if he yells at the top of his voice no one can hear him, he is obliged to proceed with burying this corpse. This ruling is based on the Talmud, tractate Yevamot folio 89: ”If the High Priest calls out in order to attract the attention of other human beings, and no one answers his call, this is the situation known as מת מצוה, “a corpse that anyone is called upon to bring to burial forthwith.” This is a decree of great severity, i.e. if there is anyone else available the High Priest must not defile himself in order to perform this last act of kindness for the dead.