Ya this definition from the holy Wiki for incest.

"Incest refers to any sexual activity between closely related persons (often within the immediate family) that is illegal OR socially taboo".

I'm surprised no one here has mentioned the term "blue blood" which is what royalty is often referred to, and is believed to come from a genetic blood disorder that was abundant in royal families in the past, due to closely related family members marrying in order to keep the royal line "pure".