From: Rabbi Moshe Sofer
To: Rabbi Simcha Noson Ellenberg
The Chasam Sofer dismantles a sfek sfeika argument to permit marriage with one's father-in-law's wife, showing that Ashkenazi communities have definitively accepted Rabbeinu Tam's prohibition and that the second doubt lacks any authoritative resolution. While his theoretical analysis suggests the prohibition — rooted in mar'is ayin — may lapse after the wife's death, he explicitly declines to rule leniently in practice and defers to greater authorities.
Source: Shu"t Chasam Sofer EH I §38