We have two things here: morality and manners. Morality is universal at its core. Yes, some details may vary, but morality is universal. For example, cheating, lying, stealing, unjustly killing, etc are universally recognized evils. Goodness, honesty, generosity, etc are universally recognized virtues.
Manners can change from time to time and place to place. How you respect others can vary based on your culture and society. Islam says you can take manners from society, but morality should be based on a fixed foundation from Allah. We cannot let society decide morality for us, because society is ever changing and doesn’t have a fixed foundation.
Now as for incest, we find that our Fitra (internal disposition) is repelled by it. If you examine societies around the world and throughout history, you find that most societies rejected it, whether they were religious societies or secular societies. Most animals are even repelled by incest and don’t commit it (so you don’t see the offsprings mating with their parents). So we can say that the evil of incest is universally recognized (even if a few see it as ok).