About incest, the most acepted teory about why it is taboo is because of economic reasons (Levi Strauss): forming links outside of the family core to increase survival... but of course there are more anthropological reaosns (like Westermarck, Marvin Harris, Freud, etc.). Biologically speaking if the genetic pool is too big the results of incest probably would be harmful (like what happened with the spanish monarchy) . On the other hand, for small genetic pools incest is perfectly fine even beneficial.
Historically speaking the taboo of incest have existed in every culture BUT is different from one to another depending of the cultural view of kindred. For example, one can come across a culture were having a relationship with your aunt may be taboo but not with your sister.
Culturally speaking Hari and Eugene having a relationship may be a problem bc they are clearly living in an european Early Modern Era setting. Biologically speaking these two don't share the same blood in asociety were the genetic pool is really big, so theres no problem in that part.
Personally speaking I don't think there's a problem with these two ending together.
Tey're pretty close in age, it seems they respect each other and they don't follow each other blidly.
But this is only my opinion, also I never intended for this to be so long OMG