THEOSISTheodoret of Cyrus
This represents the utmost seriousness of wrongdoing. Engaging with another man’s wife is not equivalent to tarnishing the sanctity of one's own son's relationship; just as adultery is a graver sin than fornication, so too is the act with one’s daughter-in-law even more sinful than adultery. Anyone who has a sister, particularly one who is his father's daughter, shall bring a curse upon you. Here, the severity of the sin is evident as he illustrates it through the case of a husband and wife who share the same father. Following this, he continues to address the sin of greed.