"Yes: It is so wrong. Question answered, can we go home?" XD
Added the conspicuously missing incest tag.
To be pedantically clear: Outside of making fun of this for the specific-questionable-fetish fodder that it is, there are of course a nuanced arguments to be made about how bad incest is (e.g. most of us start to get unsettled around the distance of first cousins, do step-siblings count, which bits are "eww that's vile" wrong vs "people are going to get hurt and that's not okay" wrong) and under which circumstances we should or shouldn't give people leeway to just do their own thing, and for what reasons... But those do not on the surface of it appear to be the sort of deep questions this particular story is setting out to address. XD