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

Second Section – the Negative Commandments

115. It is a negative commandment “do not have relations with a daughter-in-law, as she is his son’s wife”

As it states in Vayikra (Leviticus) 18:15), “do not uncover the nakedness of your daughter-in-law.” This applies if the son is alive – even if he has divorced her – and also after the son is dead. If there are witnesses and warning of the punishment, then this person receives the death penalty of stoning. If it is done with intention but there was no warning, he receives spiritual excommunication. If it is done in error, he is required to bring a sin-offering. Through the Sages’ Laws of “Shnius”, this includes his son’s daughter-in-law and down through further generations. It also includes the daughter-in-law of one’s daughter but this is not extended through further generations.

This applies in all places and at all times.