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

114. It is a negative commandment “do not have relations with one’s father’s brother”

As it states in Vayikra (Leviticus) 18:14, “Do not uncover the nakedness of your father’s brother.” This is in addition to the prohibition of having relations with another man. If a person has relations with his father’s brother with witnesses and is given warning of the punishment, both of them are to be given the death penalty of stoning. If there was no warning, but the act was done intentionally, he is spiritually excommunicated. If the act was done by mistake, he is required to bring two sin offerings (one for this transgression and another for transgressing the commandment of having relations with another man.)

This applies in all places and at all times.