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

135. It is a negative commandment “a non-relative shall not marry the wife of a man who passed away without any children”

As it states in Devarim (Deuteronomy) 25:5, “the wife of a man who has passed away without children may not marry a non-relative.” If the man who passed away had a brother from his father – if the brother did not perform chalitzah [the ceremony to allow the woman to marry anyone], then if the woman marries a non-relative and has relations with him, both of the people have transgressed this negative commandment. He is required to divorce her, then the brother must give chalitzah, and then she may get married to a non-relative (see Positive Commandments 46)

This applies in all places and at all times.