>>936855674 (OP)The inherent power dynamics of the family
Consent requires the freedom to say no without threat of consequence
If you rely on your family for anything, which most people do, then your ability to say no freely is compromised. You're forced to wonder if saying yes or no will in any way alter your relationship with that family member, whom you rely on
It's too hard to codify into law a metric by which one could know whether it's possible in a given family for consent to be freely given, so a blanket ban is reasonable, because at least it's enforceable
That said, it's at least theoretically possible for consent to be truly given, in which case it would be morally fine
There's nothing inherently morally wrong with related people fucking if it's really consensual, it's just so hard to know whether it's really consensual that making it illegal is the sensible response