She is your mother: Incest was illegal because it disrupted families. The commands about this sin included close relatives, even if not related by blood. Incest was a serious crime because it caused competition and chaos in the family. It threatened home safety by breaking family intimacy and belonging.
The Bible directs this command to men, even if the woman was older, like a mother. Men, especially household leaders or firstborns, had more power. This power came with the duty to use it lawfully. Men were more often the sexual aggressors in illegal unions, though not always (for example, Genesis 39:6–18).