Do not be sexually intimate with one’s daughter-in- law means his sons wife
“The nakedness of your daughter-in- law you shall not uncover” Vayikra 18:15
In his son’s lifetime, even if he divorced her and after his son’s death.
Where there were witnesses and a warning both deserve death by stoning.
If without a warning and done deliberately the punishment is spiritual severance.
If done unwittingly a sin offering are required.
The secondary sexual relations prohibited by the Scribes are
the daughter-in-law of one’s son’s son with no limit to future generations and
Only the daughter-in-law of one’s daughter
This applies in all places at all times.