The Torah in Parashat Ahareh-Mot (Vayikra 18:17) commands that once a man marries a woman, he may never then have relations with or marry her daughters or granddaughters, even those produced with a different man. The Sefer Ha’hinuch lists as the 203rd Biblical command the prohibition against relations with one’s wife daughter; as the 204th Biblical command the prohibition against relations with one’s wife’s son’s daughter; and as the 205th Biblical command the prohibition against relations with one’s wife’s daughter’s daughter.
The Sefer Ha’hinuch, in discussing this 205th command, summarizes the various family members of one’s wife whom one may not marry or cohabit with. On the level of Torah law, the Sefer Ha’hinuch explains, six of one’s wife’s relatives are forbidden – three “above” and three “below.” The three relatives “above” are the wife’s mother, and both her grandmothers. The three “below” are the wife’s daughter, her son’s daughter, and her daughter’s daughter. The Rabbis added a safeguard, forbidding an additional six relatives – four “above” and two “below.” The four “above” are the wife’s four great-grandmothers, and the two “below” are the wife’s great-granddaughters – the granddaughter of her son and the granddaughter of her daughter.
These prohibitions apply in all places and in all times. If one had a relationship with his wife’s granddaughter, from either her son or her daughter, while the wife was still alive, then both he and the woman he cohabited with are liable to Serefa (execution by fire). If the wife had already passed away, then the punishment is Karet, and if this was done unintentionally, then the violator must bring a Hatat (sin-offering). If one had intercourse with a family member with whom the Rabbis forbade relations – such as his wife’s great-grandmother or great-granddaughter – then he is liable to Malkut for transgressing the Rabbis’ edict.