Sefer HaChinuch ספר החינוך

The nakedness of the wife of the brother of your father, you shall not reveal: Not to have intercourse with the wife of the brother of your father. And this one is called his aunt (dodah) in Scripture, since she is the wife of his uncle (dod), as it is stated (Leviticus 18:14), “to his wife you shall not approach, she is your aunt (dodah).” And this warning is from when she was betrothed to his uncle — meaning to say, she was designated for him — and there is no need to say, after she married him. And [this is the case] whether [it is] in the lifetime of his uncle and even after he divorced her, or whether after his death (Yevamot 97a).

We have written above about the roots of the commandment (Sefer HaChinukh 190).

The laws of the commandment: For example, that which they, may their memory be blessed, said (Yevamot 54b) that he is called the brother of his father regarding this prohibition of his wife when he is the brother of his father from the father’s side. But the brother of the father from the mother’s side is not [included] in this prohibition of “to his wife you shall not approach” by Torah writ. However the Sages made a fence and forbade even the wife of the brother of his father from the mother’s side, in order to distance us greatly so as not to touch a prohibition of the Torah. And likewise they also forbade the wife of the brother of his mother — whether she is a sister from the father’s side alone or from the mother’s side alone. And even though the wife of the brother of his mother is not forbidden at all from Torah writ; since the Torah forbade the wife of the brother of his father from the side of the father — who is called “his uncle” — they added upon the prohibition to make a fence with all of these that are similar to it.

And this prohibition is practiced in every place and at all times. And [in the case of] one who transgresses it and laid with the wife of the brother of his father from the father’s side, during the lifetime of his uncle, volitionally — and there were witnesses — the death of both of them is with strangulation, as this is the law of the one who has intercourse with a married woman, with strangulation. And if he laid with her volitionally after the death of his uncle, he is liable for excision; and if there are witnesses, he is lashed; inadvertently, he brings a fixed sin-offering — as it is well-known to us that with anything for which there is excision if volitional, its inadvertent transgression is with a sin-offering. And if he laid with the wife of the brother of his father from the mother’s side or the wife of the brother of the mother — whether from the father’s side or from the mother’s side — we strike him with lashes of rebellion when volitional, and he is exempt when inadvertent.