I am not a young earth guy, and its the type of incest. Canaan, Moab, and Benammi were all the product of parent/child incest.

11 And the man that lieth with his father's wife hath uncovered his father's nakedness: both of them shall surely be put to death; their blood shall be upon them

I disagree 9n Jonah. The mosaic covenant is the entire point of the OT. God punished the children of Israel for failure to keep the law. He redeemed th through the law. While His desth freed us from the law, they were nevertheless bound to it. If ruth were a moabite, the moment sue entered the congregation of Israel, she should have been cast out with her husband.