@Yansas Actually first cousin marriage is legal in most countries, in Europe, most of the Middle East and Asia (not in China though) and all of America (although in the USA it varies state by state from being legal, to first cousins not being legal but further cousins being okay, to being a criminal offence in some states).
As our understanding of genetics and the consequences of inbreeding have increased, it has steadily become a moral taboo and the practice has nearly disappeared in many countries where it used to be widespread in medieval times as you say. But technically there's nothing preventing it (except the dirty looks of neighbours who learn about it) in most of the world.
With all that said I am very glad that the practice is not common, it's too close to incest. I do feel sorry for Leon though, his love is doomed.
On another note why do these men feel like when a woman rejects their help and protection (because she doesn't need it) they feel like she's rejecting them on a deeper level? Seriously, chill, she can take care of herself, and she's not rejecting your feelings, she just doesn't need you acting that way.