The nakedness of your father’s wife you shall not uncover… (Leviticus 18:8)
We mentioned in the previous mitzvah that it is prohibited to sleep with one’s mother (in the sexual sense of the term) even if she is not married to one’s father. Here we see that a father’s wife is equally prohibited, even if she is not one’s mother.
While the threat of birth defects due to inbreeding does not apply in this case, most of what we said regarding incestuous relationships does. For example, one is supposed to honor stepparents and this is not honor. (It’s certainly not honoring the father! If you disagree, look at the incident of Avshalom with his father David’s concubines in II Samuel chapter 16.) This law applies even if the father divorces her or passes away. Once he has betrothed a woman, she is forbidden to his sons in perpetuity.
Unlike one’s mother, there is no prohibition on marrying a stepmother’s mother. If you’re inclined to do so, go ahead, though be advised that this is the plot of the song “I’m My Own Grandpa.” It is, however, rabbinically prohibited to marry the wife of one’s grandfather, even if she is not his grandmother.
As mentioned, if one has relations with a woman who is both his mother and his father’s wife, he would violate both Mitzvah #190 and Mitzvah #191.
This mitzvah applies in all times and places. In the Talmud, it is discussed in the tractates of Sanhedrin (54a) and Yevamos (21a-22a). It is codified in the Shulchan Aruch in Even Ha’Ezer 15. It is #331 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #113 of the 194 negative mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.