One who marries a woman and then has relationships her father's mother or her son's daughter gets burned to death by Beth Din.
If his wife died and then then has relationships with her father's mother or her son's daughter he gets Karet.
By Rabbinic decree it's forbidden to have such relations even if neither women are married to him.
By Rabbinic decree this applies to great-grandchildren as well.
Applies to everybody, everywhere, always
Verse: "...her son's daughter..." (Vayikra 18:17)
Source: The Chafetz-Chaim's Sefer haMitzvot haKatzar; Prohibition 122
Tuesday, 17 Tamuz 5770