A more thorough explanation:

Definition: Incestuous adultery is a type of adultery where the people involved are closely related to each other. Adultery is when a married person has sexual relations with someone who is not their spouse. In some places, adultery is a crime, but it is rarely punished. If a couple gets a divorce, proof of adultery can be used as a reason for the divorce. If one spouse commits adultery, a court may also use it as a reason to give them less property in the divorce settlement. In the past, judges would often use adultery as a reason to deny the offending spouse custody of their child, but today, only the effect on the child is considered.

Examples: An example of incestuous adultery would be a brother and sister having sexual relations with each other. Another example would be a father having sexual relations with his daughter. These examples illustrate the definition because they involve people who are closely related to each other and are committing adultery by having sexual relations with each other even though one or both of them are married to someone else.