The Sharia of Moses in the TORA
Part 2 - The 365 Prohibitions of the Tora
333 - HAVING INTERCOURSE WITH THE DAUGHTER OF ONE'S FATHER'S WIFE IF SHE BE HIS SISTER
One who violates this prohibition by having intercourse with a sister, whose mother is married to his father, is liable to extinction, but only if his sin is willful. If he sins unintentionally, he must offer a fixed sin offering.