Western society was mostly formed by Christianity. Christianity used to say "when you marry someone, you marry their family", meaning that as far as Christianity was concerned, upon marriage, both side became fully related.
This was done away with in most if not all denominations around 500 years ago. Since Catholism is so nice and organized, we know they changed it during the council of Trent, halfway through the 16th century.
Good thing too since that's an absolute moronic definition of incest. ---