Sefer Ha’mitzvos Ha’Katzar (The concise book of mitzvos)
By the Chofetz Chaim

This book lists the Torah mitzvos that can be observed today

First Section – the Positive Commandments

59. A Cohain is required to make himself spiritually impure by taking part in the burial of his close relatives.

As it states in Vayikra (Leviticus) 21:2-3, “for his mother, his father, his son, his daughter, his brother, and his unmarried sister… he shall make himself spiritually impure.” A Cohain may not become spiritually impure for other people, but for these close relatives he is required. Also, he is required to become spiritually impure for his wife. For his brother and sister, it is only if they are from his father. A woman is not required to become spiritually impure for her relatives, but she is allowed to. Either way, she is required to mourn for them. This positive commandment includes the commandment of mourning: every Jewish person is required to mourn for his or her seven relatives mentioned above. The first day of mourning is from Torah law and the other days are Rabbinic law.

This applies in all places and at all times.