A cousin is someone that everybody knows you are related to but is someone that you really do not want to admit to being a relative. This can be awkward when your parents always refer to you as their "little cousin".
Cousins can be blood relatives or not and people quite distantly related can be considered cousins. It all depends on how many brothers and sisters your parents have, either by birth or kidnap. Lots of cousins make it possible to mooch meals from every member of your extended family and is a good thing so long as you keep your own address a secret.
While hillbillies and royalty are always marrying their first cousins, it is often illegal to do so in many US states. This is because those places have enough sullen drooling inbreds peering out of windows and playing banjos on porches, thank you. Texas, for example, allows marriage with porcupines and cacti but not human first cousins. Unfortunately, everyone in Texas is closely related to everybody else from being together in the Alamo. This makes legal marriage so difficult that porcupines and cacti are in short supply.
In the case of royals, they have large castles with underground chambers in which to throw the worst of the family, with or without banjos. Plus nicely tailored uniforms can make grotesque royals with bulging eyes and asymmetrical heads look really good, especially when they're not swinging from chandeliers and stuff.