Most definitions I found link the word incest to "too closely related to marry", which makes it a political term, really. There's a lot of grey area there.
When it comes to nuclear family relationships, the immediate blood relation, family dynamic, and other things come into play. In sexual relations such as these, there is usually a powerful figure dominating a weaker partner. That in itself leads to problems regardless of potential birth defects.
In most places in Europe and roughly half of America, even first cousins aren't considered incest by that definition, as it is legal to marry a first cousin and the birth defects (despite much ignorance surrounding the subject by "ewwwww, gross" intellectuals *sic*) are barely higher than normal and roughly on par with a woman in her late thirties having a child with someone completely unrelated to her.
By that definition, second cousins are entirely off the incest list.
So, again, it really depends on what you mean by "incest".
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