17 And if a man takes his sister, daughter of his father or his mother, and has sex relations with her and she with him, it is an act of shame: they are to be cut off before the children of their people; he has had sex relations with his sister, and his sin will be on him. 18 And if a man has sex relations with a woman at the time when she is unwell, he has seen her fountain and she has let the fountain of her blood be uncovered, and the two of them are to be cut off from among their people. 19 And you may not have sex connection with your mother's sister or your father's sister, for they are his near relations: their sin will be on them. 20 And if a man has sex relations with the wife of his father's brother, he has put shame on his father's brother: their sin will be on them; till the day of their death they will have no children. 21 And if a man takes his brother's wife, it is an unclean act; he has put shame on his brother; they will have no children.
Matthew Henry's Commentary on Leviticus 20:17-21
Commentary on Leviticus 20:10-27
(Read Leviticus 20:10-27)
These verses repeat what had been said before, but it was needful there should be line upon line. What praises we owe to God that he has taught the evil of sin, and the sure way of deliverance from it! May we have grace to adorn the doctrine of God our Saviour in all things; may we have no fellowship with unfruitful works of darkness, but reprove them.