In the context of the prohibition against having intimate relations with one’s wife’s daughter (even though the daughter was fathered by a different man), the Torah forbids also relations with one’s wife’s daughter’s daughter (Vayikra 18:17). The Rabbis enacted a safeguard to this prohibition, forbidding relations with one’s wife’s great-granddaughter, specifically, the wife’s son’s granddaughter.
This prohibition applies in all places and in all times. If somebody married a woman and then had relations with her granddaughter, then if the relationship occurred during the wife’s lifetime, the man is liable to Serefa (execution by fire). If the wife had already passed, then the punishment is Karet. If one transgressed this prohibition mistakenly, then he must bring a Hatat (sin-offering). If one violated the Rabbinic enactment, having a relationship with his wife’s great-granddaughter from her son, then he is liable to Malkut for transgressing a Rabbinic edict.