Do not be sexually intimate with one’s mother
“The nakedness of your mother you shall not uncover” Vayikra 18:7
Even if she was not your father’s wife (-113) but she was his mother through immoral relations, she is still his mother.
It makes no difference if he was sexually intimate with her during his father’s lifetime or 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.
If she was also the wife of his father he would need to bring two offerings
The Scribes instituted a law called “Rabbinic secondary relations”
This forbids sexual intimacy with
ones mother’s mother etc. with no limit to this prohibition going back through earlier generations.
Only the mother of his mother’s father.
Father’s mother etc. without limit
Only the mother of the father’s father.
This is in effect in all places at all times