I'm only asking out of pure whim since, a fifth cousin would fall under the five or six generations gap. Inbreeding isn't really frowned upon because of genetic anomalies, although biological diversity is most certainly a legitimate reason to not do so. It has a lot more to do with the other part of gender roles that aren't discusses, specifically as related to interfamilial dynamics and responsibilities. The zeitgeist is dominated by anecdotes of parents raping their children, at once highlighting moral decay while also highlighting the collapse of society. In reality, inbreeding is far more common than parental sexual abuse and entail a more complex layering of factors, which is what the son is bringing up by pointing to the taboo narture of it. Like it or not, the discussion has a lot more to do with broken families (the adoption that made the lineage murky in the first place) and the responsibilities older generations toward their younger progeny. and its not a comfortable or easy discussion. The parents are thrusting this on their children, but that doesn't mean it's not worthwhile discussing the direct link between societal breakdown and this situation.