The 339th prohibition is that one is forbidden from having relations with a woman and her daughter's daughter.

The source of this commandment is G‑d's statement,1 "[do not commit incest with a woman and her daughter. Do not even take her son's daughter] or her daughter's daughter."

For this prohibition the punishment is kares and s'reifah for an intentional transgression, and a sin-offering if unintentional.