Do not be sexually intimate with your father’s wife.
“The nakedness of your father’s wife you shall not uncover” Vayikra 18:8
Even if she is not your mother, if she is your father’s wife through betrothal or marriage,
Both in the father’s lifetime, even if he divorced her and after his death.
Where there were witnesses and a warning both deserve death by stoning.
If there were no witnesses and done deliberately the punishment is spiritual severance.
If done unwittingly a sin offering is required.
The secondary sexual relations prohibited by the Scribes are
the wife of one’s father’s father with no limit to earlier generations and
Only the wife of one’s mother’s father
This is in effect in all places at all times