The 343rd prohibition is that one is forbidden from having relations with one's son's wife.

The source of this commandment is G‑d's statement,1 "do not commit incest with your daughter-in-law."

For this prohibition the punishment is stoning.2 If the act was intentional, but there was no valid testimony or no one knew of the transgression, the punishment is kares. If it was done unintentionally, a sin-offering is brought.