Do not be sexually intimate with the wife of one’s brother, whether from the same father or mother and even from immoral relations
“You shall not uncover the nakedness of your brother’s wife” Vayikra 18:16
If one was sexually intimate with her while his brother is alive and he had not divorced her both are to be executed by strangulation as she is a married woman.
If the brother had divorced her or he was sexually intimate with her after the brother’s death, not for the purpose of a levirate marriage-
If done intentionally they deserve spiritual severance
If done unwittingly a sin offering is required.
If she is his brother’s childless widow with whom he has performed Chalitza / Removal of the shoe ceremony, then this does not deserve spiritual severance but transgresses “he shall not build up his brother’s house “Devarim 25:9
This applies in all places at all times