I don't see any problem with it as long as they didn't have kids. But since they did not it is a problem. The laws against inbreeding are there for a reason. If you have a genetic defect or bad recessive gene it is more likely to get expressed if you mate with a relative. Hence why some small Native American tribes have major birth defects. I'm Native American (3 tribes) so I can say this ok.

Its true the small tribes whose elders push to marry within the tribe end up creating problems. The gene pool is too small and after a few hundred years everyone in the tribe is inter-related. Though its nice for the kids to be able to say they're fullbloods its not healthy unless their parents are from different reservations if the tribe has more than one. The old Clan rules of some tribes tried to deal with this genetic situation by tabooing inter-clan marriages, but today that's just not enough.
My great-grandparents found out after they were married that they shared a cousin so apparently they were related. Doing genealogy research I see where the families crossed several times. They had no idea at the time they might be cousins though. Well I can tell you about half of my family is on the spectrum. I don't stand out as an Aspie in the family because everyone else is pretty much the same way, diagnosed or not. Now I have a multitude of health problems including autoimmune illness, PDD and AS, congenital heart defect & unexplained hearing loss. My brother and a cousin were both birth defected and died. Course it didn't help that mom and aunt were given DES. My cousin's son is a major Aspie and he was born missing bones! They x-rayed him when he was a little kid and the dr's like "Uhmm did you know your son's missing some bones." Several family members have had brain tumors, adrenal problems and congential heart and sinus problems. So you can't tell me inbreeding is ok.
That must really suck to have so many relatives afflicted with so many disabilities. All the more reason why incest should be prohibited.