Shalom David,

1. I'm glad to discuss these serious issues with you in the context of learning, but of course for a practical ruling in a real life scenario, it would be best to connect with a competent Halachic authority.

2. The Rambam discusses these issues in chapter 2 of Hilchos Isurei Bi'ah, so that is a good place to research for more details. Generally speaking, however, when a man marries a woman, a number of her relatives become forbidden to him forever, meaning even after she dies or they get divorced. The specific relatives are: her mother, her parents' mothers, her daughter, and her children's daughters.

3. Intimacy with any of them while his wife is still alive would warrant capital punishment, Sereifah. But if his wife has already passed away, it is Asur but there is no death penalty (the Rambam maintains there is Kares, but other Rishonim disagree.)

4. His wife's sister, notably, is forbidden only while his wife is alive. After his wife passes away, he may marry the sister of his departed wife.

5. Note that all of this applies only to relatives of a man's genuine wife , meaning he actually married her. If, however, a man engaged in a promiscuous relationship with a woman, that woman's relatives do not become forbidden to him mid'Oraisa. Nevertheless, Chazal enacted that even in such a case he should not marry them during the woman's lifetime.

6. Note also, of course, that not all of the wife' relatives become forbidden. For example, the husband is permitted to marry the daughter of his wife's sibling, even while still married to his original wife.

I hope this small overview is an informative start, but of course there are many details which I have not even managed to write about. I'll be happy to discuss more with you if you have follow up comments or questions.

B'Verachah,

Yishai Rasowsky

This is not a Psak Halachah.