Well, honestly, there's a research about the attractiveness of someone with the closer DNA with themself, which is, their blood related family (in extreme case, it's incest, either child with parents or siblings). The closer your DNA with someone, they might have a more distinctive similarity theory the said person. Either its facial it personality (you might realize that at some point your lover have something that looks similar to your face, either it's nose or eyes).
In Asians country, it is still pretty common to marry your cousin (its pretty no no in Europe and U.S., I suppose)
The research also shows that marrying your same blooded family will increase the possibility to carry inherited diseases, and there might be some abnormalities within your offspring at some point.
The only thing preventing this to became full incest between humans is social impact. Your surroundings didn't accept this kind of incest behavior, so it didn't happen often.
But if you ask me, I have no idea since I'm not a researcher. ╮( ̄▽ ̄)╭
I don't understand something. Or rather, more like I do not know anything about it... so, here's my question. Is it normal, or at least, more common to fall in love with your blood related familymember in Japan? It just seems so common in manga and anime and I do not understand how you could be attracted to someone of your same blood. I do not judge, but I'm just saying that I do not understand it, and it seems so common in manga I am just wondering if it is just a manga thing like so much else or if it reflects something in their society. Is it the taboo thing and closeness that is appealling or?