Leviticus 18:17 - Ellicott's Commentary On The Whole Bible

A woman and her daughter. — That is, if a man marries a widow who has a daughter by a former husband, or if he forms an alliance with a woman who has a daughter out of wedlock, he is forbidden to marry also the daughter. But though this prohibition is directed against a peculiar form of polygamy. there can hardly be any doubt that, as the administrators of the law during the second Temple interpreted it, if he married either of them and she died, he could not marry the other any more, and that this prohibition did not apply to cases of illicit commerce. Criminal intercourse with one did not preclude him from marrying the other. For contracting the kind of polygamy here forbidden, the offenders were punished with death by fire. (See Leviticus 20:14.)