think about if it happened irl
spoiler alert: bad
Only bad, based on social opinion.
Even the claims of medical or genetic issues occuring with incest only comes up in rare cases when the situation is going on for multiple generations. Just one or two generations doing it doesn't cause such issues to occur as some people claim.
Plus, those issues would only occur if there is a child born from such relationship. If no child is born, then there is no point of talking about possible genetic issues.
Also, Incest had been part of many cultural myths as well.
Like if you consider the story of Adam & Eve or Noah's Ark, and believe them to be true. Then it's clear that every human on earth is a descendants of someone who was born from an incest relationship.
Even in nature, Incest is actually pretty common, though info about that is either usually suppressed or portrayed as something negative based on some some vague research result.