Now, this presents incestous marriage as sin, but so is marriage to an unbeliever.
These are two different categories. An incestuous marriage makes the marriage a sin. In the case of a believer marrying an unbeliever, the believer personally breaks God's word which prohibits being unequally yoked, but the marriage itself is lawful because marriage is a creation ordinance and honourable in all.
The incestuous marriage should be annulled.