Oh I know, but that's the thing about Hetalia; a lot of what nations do is metaphorical, and I think there's a strong case that this was one of those times. Lets say they are brothers genetically; How did two separate quasi-immortal beings that came about in different times that span hundreds of years end up genetically similar enough to constitute "brothers" on a DNA test? And why does that not apply to Denmark+Sweden as well?

As other anon said Himaruya is very inconsistant, and personally I think if he means that sibling nations are literally related he has a lot of 'splaining to do.