About that smell comment.

There are several registered cases of genetically related people that, unknowing they are related, develop attraction and engage in sexual activity. Some cases even mention the couple being more attracted to each other due to scent.

There’s a small article in Wikipedia here.

Now, for an anecdote: I knew a couple that was genetically related and they only discovered they were siblings because they tried to reconstruct their family tree through official records and things came out off.

They already had two perfectly healthy children when I met them. They kept their blood connection as private as possible but they intended to tell their offspring the whole story when they could understand all of it and its implications.

Now about the incest taboo: I find no issue in consenting adults engaging in sexual activity. Normalizing it, I find deeply troublesome. It would completely inbalance relatioships between family members and wreak havoc in the gene pool.