I would perhaps suggest @Cody_G that your examples might not follow as logically from your example from Cain as they might appear.
Most would say that issues around food laws and sabbath regulations relate to the keeping of ceremonial laws. I’d also point out that how Christians are to relate to the Sabbath laws is an extremely contested issue which is very complex, and wide ranging in the views held by Christians of all stripes today.
Sexual ethics, in this case incest, are governed by the moral law, which if anything becomes stronger in the New Testament. For example, polygamy is not explicitly condemned in the Old Testament, where as it is in the New Testament (An elder/overseer must be… “a husband of but one wife” 1 Tim 3:2, Titus 1:6 (ESV).
The problem of incest is further compounded in that there would need to have been generations of son-sister or worse father-daughter or mother-son sexual relationships before one could even legitimately pursue cousin-cousin marriages. Either way, incest among members of the nuclear family would have had to go on for sometime before there was a big enough population for Cain to found anything that remotely resembled a city in Nod (even by ANE standards). That might sound shocking, but the suggestion we are considering is that Cain and his sister-wife are the only two people living in Nod at this time. Personally, I think the simpler option is to suggest that Cain married into a well established tribe in Eden and migrated them to Nod, or married into a nomadic tribe in Nod and convinced them to settle down.
The point is the plain reading of the text neither supports or invalidates either mine or your position because it is silent on where Cain found his wife:
Genesis 4:16-17:
So Cain went out from the Lord’s presence and lived in the land of Nod, east of Eden. Cain made love to his wife, and she became pregnant and gave birth to Enoch. Cain was then building a city, and he named it after his son Enoch.
I’m sure we can agree that the text above says nothing about a sister leaving with Cain and it says nothing about Cain’s family relation to his wife. Neither does it say anything about Cain meeting a women near Eden or in Nod. Where Cain found his wife is not the concern of the text.
So, can we agree that whether Cain married his sister or married a human women not of Adam’s line, we are both making an appeal to information not found in the text itself? Can we agree on that at least?