Q. Leviticus 18:9 says that having sexual relations with a sister is forbidden by God, yet Abraham did. He married his half sister; yet he was considered righteous by God. How could that be?
A. Yes, Abraham married his half sister. They had the same father but different mothers (Genesis 20:12). The commandment prohibiting marriage to a sister didn’t come until several hundred years later. But irrespective of that, Paul wrote that Abraham’s righteousness was imputed to him by faith (Romans 4:3).