After seeing this topic I did a bit reading on this the past few days to get a better understanding of at least the physical aspect regarding this.
A lot of what I wrote here came from various sources online that seemed reputable (articles by gentile researchers on the subject), I.e, there was none better I could find, but still, take it with a grain of salt since this subject is still barely understood even though scientists like to pretend they know it well, and various factors are ignored by the science (such as race, unsurprisingly).
It was surprising to say the least how little risk there can be on the physical/genetic aspect, but that only reinforces the limitations of today’s material science, as it doesn’t provide any reliable guidance to follow on this.
Just because the physical risk can be low, doesn’t mean it is healthy or okay to have such relations with close relatives.
For the physical aspect, according to multiple sources, the average chance for a birth defect in a newborn among strangers is around 2-3%. So 2-3 in a hundred babies are expected to have some form of noticeable defect.
This is considered the baseline for humans. Keep in mind this does NOT take race into account, since science totally ignores race altogether on this subject and considers all humans as the same.. obviously very wrong.
What happens with incestual relationships is the risk of birth defects caused by recessive genes increases, because you have a close common ancestor and thus share more of the same genes.
A sibling will always share approximately 50% the same genes with you, and you also share 50% with your parents, since you receive half of your chromosomal pairs from each parent.
This share is on average (and due to statistical probability, usually with little deviation) divided by half with each generation or familial distances (apologies for poor wording here).
A grandchild will share about 25% of their genes with you, a first cousin (the child of your aunt), shares about 12,5% of their genes with you, and a second cousin (a person who’s closest common ancestor with you is a great grand parent) shares about 3,1% of your genes with you on average.
What this means in practice is, in the absence of a known recessive genetic condition, the chance for a child to have a birth defect if you have kids with your first cousin, is only 6-8%, and your second cousin is only 3-4%, from what I read.
Barely any higher than the human baseline when you reach the second cousin level, and surprisingly low in case of first cousins.
In case of known recessive genetic conditions, this chance can be exponentially higher, you can find out exactly how high by comparing your genes with theirs and seeing if you both carry the same recessive genetic conditions.
People may carry multiple faulty recessive genes, and all of them can carry over, the chance you both have them becomes increasingly smaller the more distant you are from the family member in question, so the risk for carry over and expression of these conditions becomes exponentially smaller the more distant you are until you reach the established human baseline.
Other complications can still happen though, as you share more genes with your family members, other genetic traits can be reinforced over time, which sometimes can be a good thing as well (such as if you have certain genes that give strong resistance to certain diseases in your family), but if you have genes that negatively affect fertility, or other things, this can become reinforced over time as well.
The main risk lays in the fact that overtime, more and more negative recessive genes have the chance to be inherited, and you get a case of Habsburg syndrome in your family, where everyone looks like an ugly gremlin with too many defects to count (the fact they mixed with jews only amplified that of course, since the genes of Jews are a literal cesspool).
At the point of half 4th cousin, a person who’s common ancestor with you is a single great-great-great grandparent, the chance for birth defects is about the same as with a stranger, as they only share about 0,25% of your genes with you.
Anything beyond that has no effects at all in practice, and I’m fairly certain there would be no spiritual problems from that either since you basically never even know any cousins beyond your 3rd, if even your 3rd (I’ve never even heard of my 3rd cousins, they are basically strangers to me, even my second cousins are, and I have dozens of them, so it’s possible I met some cousins without ever realizing due to how big my family is).
A safe cutoff point to consider would be beyond 4th cousin, both physically and spiritually. Possibly 4th cousins or even 3rd cousins can be fine, but 3rd is still questionable to me.
I do not know all the spiritual implications and aspects of this, as there is little to no information on this available, so I can only go by feeling/common sense on that.
Something interesting I experienced on this, is I found a person whom I know had been my sibling in a, or possibly multiple past lives.
Same as how in healthy adults, one would never have sexual desire towards ones sister, that same feeling still remains with me today regarding them, even though we are completely unrelated in this life, and she is certainly a very desirable person.
Especially after we confirmed with nigh certainty we had been siblings, I have felt no different than a brother to her, even though in this life we are only friends.
Once a sibling, always a sibling, is what it feels like to me, and the implications of incest would carry through as well, even if purely physically speaking there would be no issues in this lifetime.
The spiritual bonds you make through being family with someone run very deep, especially if you both are spiritual people, advancing your souls through your lifetimes.
Something to note, in the case a person has totally pure genes, with no negative recessive genes at all, something not possible on earth as of now, but very likely for our Gods, there can never be any genetic birth defects at all, even if they would have children with their siblings.
So far as I understood, the Magnum opus also serves to perfect and clean up your genetics besides all the other effects it has.
If people complete this in the future, it would likely eliminate any faulty recessive genes from their genome, and eliminate the risk for birth defects to occur.
It is at that point the only problems that arise from incest are the spiritual ones, however, if you are that advanced spiritually, you would know better than to sexually desire your close family, so it might not be relevant anymore.