Leviticus 18:18

18:18 You must not take a woman in marriage and then marry her sister as a rival wife while she is still alive, to have sexual intercourse with her.

Matthew 19:5-6

19:5 and said, ‘For this reason a man will leave his father and mother and will be united with his wife, and the two will become one flesh’? 19:6 So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate.”