@Fedelolo
Stats disregarded, check the science and the biological issue with couples between cousins.
Check the history of elitism and egomania that such relationships are code for.
Check the actual number of cultures worldwide who actually approve of such relations.
Moreover we're talking about first cousins here, not some type of distant relative.
And make no mistake, the cousins thing is only one of the many issues with the idea of this girlfriend. Being underage is one thing, but this girl is twelve, potentially even younger should she have skipped a grade (which might be implied since that would make ~4 year old Rentarou cable of tackling a grown man out of the way). At twelve, her puberty has not reached the point where her own feelings on love would make any sense to her, so her opinion on it shouldn't be trusted as her own, even if it come straight from her mouth.
And then there's the thematic issue with all of this as well. Literally, their entire love story was set up by someone else, not in a hand wavy, up in the sky, "god did it" manner, but in a tangible, creepy-uncle type way.
Thinking it over, I'm confident that the plot will turn a comedic direction about all of this, and either EVERYTHING will have been due to the uncle, or this is going to smoothly transition into a plot to, as I said, go against the God of Love from the beginning of the series, and to thematically discuss all of the implications in that.