One thing you’ll note if looking at a genealogy of the Greco-Roman gods is that there’s a lot of incest going on in that family. Kronos married his sister Rhea, and their children included Zeus and Hera, who also married and had children. Zeus’s brother Hades married his niece, and Herakles his half-sister. This kind of thing also appears in Egyptian mythology, with the famous couple Isis and Osiris being siblings.
Interestingly, Norse mythology appears to have mostly avoided it, with the only god known to have married his sister being Njord, father of Freyr and Freyja. Mind you, with the Norse mythology we don’t have any sources as old as the ones for Greek and Egyptian, so maybe later redactors edited out the incest. The general reason for this seems to be a narrow gene pool. When the early gods wanted to marry and have children, they didn’t have a whole lot of choice in terms of mates. How could Kronos not have married a relative when all living beings were descended from his parents Gaea and Ouranos? This was less of a problem for the Norse, as they had three different divine clans to work with: the Aesir, the Vanir, and the giants. The lack of available mates often pertains to early humanity as well, as a common mythical trope is to have all humans descended from a single pair of ancestors, Adam and Eve being the most famous example. Interestingly, the Bible never specifically says that Adam’s sons marry their sisters. In fact, while Cain and Abel are presented as the only children of Adam up to that point, Genesis 4:17 brings in a wife with no mention whatsoever of where she came from. Saying that she’s his sister is an attempt to tie together what were probably totally different stories at one point. On the other hand, Abraham and Sarah, ancestors of the Jews, are specifically stated to be half-siblings, meaning that the writer of that part of Genesis apparently thought that he and all his countrymen were descended from an incestuous couple.
To examine this issue further, I think it’s necessary to look at why incest is taboo. The reason generally provided is that closely related people having children produces a genetic bottleneck, making it much more likely for the kids to end up with diseases and such that are carried in the family. If you want healthy children, it’s not a bad idea for the other parent to be from a completely different continent. Oddly, while incest is typically frowned upon, marrying within your own tribe is often encouraged or even demanded. Xenophobia is a major impediment to the development of the species. But anyway, while this was likely the reason incest was frowned upon in the first place, there are situations when it doesn’t apply. What if a brother and sister who are incapable of having children engage in sexual intercourse? And if it’s all about genetics, there wouldn’t be any problem with adopted siblings marrying and having sex, but I’m sure I’m not the only one who’s squicked out by that idea. I think it’s developed to be more about familial relationships, and how fraternal love isn’t the same as romantic or sexual love. Economics probably played a factor in the past as well, because how would the family get a dowry if their kids marry each other? The fact that even today it’s legal in some states but not others to marry your first cousin indicates that there really isn’t a universally agreed-upon line for how close a relative is too close for marriage. I’m certainly not pro-incest by any means, but from a philosophical standpoint I have to wonder how much we can rightfully enforce the taboo. We should also remember that, while most cultures forbid incestuous marriage, such is not always the case for the monarchy. In ancient Egypt, for instance, intrafamilial marriages were quite common, so it probably made perfect sense for Isis and Osiris to be married siblings. I don’t know why kings were the exception to the usual rule, but this page offers some suggestions. One likely reason is that it means less families being able to claim the throne.
In the Percy Jackson books, Rick Riordan addresses this issue from a more modern perspective without contradicting the original myths by saying that gods don’t have DNA. They must have genetics of some sort, since their children inherit some of their characteristics, but it doesn’t work the same way as human genetics. That’s also a convenient excuse for the demigods in the series to hook up with each other, even though they’re technically related through their godly parents. Percy and Annabeth Chase, for instance, are cousins once removed, but no one has a problem with their getting together.
Picture by Kara-Lija
Perhaps it also helps that Annabeth was not conceived in the usual way, her mother Athena being a perpetual virgin, although this doesn’t apply to some other demigod couples. It does seem like it’s taboo in demigod society to get involved with someone with the same godly parent, even though I don’t think it’s said to be expressly forbidden. He didn’t say as much about this in the Kane Chronicles, but I do recall a mention that the relationships between the Egyptian gods had to do with the hosts they took on, so that Isis and Osiris might sometimes have been siblings and other times a married couple. This also explains how Horus could be both Isis and Osiris’ brother and son without the need for there to be two of him.