I like how he literally calls upon Shueisha to confirm that this is acceptable. He knows that the existence of his world is 100% completely and utterly subject to the whims of the manga publisher.
Not going to comment on the relationship itself. I don't know enough about cousin-cousin relations but it is technically legal in a lot of places (and if they don't plan on having kids, there's not really anything genetically problematic about it), so... He also has a girl and her mother (and their maid) all together, so I don't think this manga cares about norms anymore.