Incest

Incest is an umbrella term that refers to romantic and sexual acts and/or relationships between people who are closely related by blood.
Many consider also acts done by anyone bound by family--including step-siblings/parents, who are not related by blood--to fall under incest, but this is technically not correct. Some refer to these cases as pseudo-incest instead, to differentiate them.

Nonetheless, incest and pseudo-incest are both considered taboo concepts and are generally universally frowned upon on Surface. In most places of the world, incest among inner family members is either prohibited by law, or even severely punished.

Consanguinity

Consanguinity is a measure of someone's relation by blood.
Grades of consanguinity refer to how many percentage of genes are shared between two people: the absolutely highest possible is 100% between identical twins, then 50% between non-identical twins, siblings and parents; 25% between half-siblings, aunts/uncles and grandparents; 12.5% between first cousins, great grandparents, etc.

Incest in Law

What degree of consanguinity is considered incest differs from country to country.
Asseculs worldwide generally allow marriages between first cousins, a consanguinity of 12.5%; while Devouts only allow marriages further apart than third cousins, a consanguinity of measly 3.125%, making it the strictest in the world.
West-Ihazon, the largest state in Ihazon, is the only place that has no restrictions on who can marry, allowing marriages of any consanguinity grade. However, marriages made in this state are not recognised in other states.
Not only that, but sexual acts between individuals of consanguinity 25% or higher are illegal in most parts of Ihazon, as well as all parts of Sas and Tekkea. While not always prosecuted, as many cases of incest go unreported, particularly severe incest cases, often involving abuse and rape of familial members, can not only land one a life sentence but even capital punishment (the latter is exempt in Tinville, as it does not have capital punishment).

Radosis does not generally consider relationships between family members illegal, but it is heavily frowned upon, even if it is somehow considered mutual.

In History

Alludes to incest have been made throughout history, as the taboo concept makes it ripe for stories of "forbidden love".
However, in the cases of acting on it, there really only exist prominent examples from Radosis.

The race of divine, who consistently made up the majority of positions of power in the early ages despite being so few in numbers, had a strong aversion to diluting their bloodline. This was further exacerbated from the Genus Reditus belief of reincarnation through bloodline, as well as the very low percentage of men among the divine population. As a result, the few times divine sought to extend their bloodline--despite the risks it carried for them--it happened with another divine, even if it meant they were closely related.
While rare for it to happen with an inner family member, there were cases of this happening even among siblings. There were also rumours that Empress Clementia even slept with her sons--though, this is not proven, and no children were conceived from it.

After the Fall of Radosis and divine were no long around, people of Radosis naturally wanted to distance themselves from them. Their often considered decadent and promiscuous lifestyle became something to avoid. The trend for divine to fall into "pure" bloodlines, often through various degrees of incest, might have made the concept highly contentious, despite--or perhaps, because of--the practice still existing today.