Can You Marry Your Cousin in Washington State?
I, honestly, hope this isn't a question you're seriously asking, but we do have an answer for you.
In Washington State, you can not marry your first cousin.
There are several states that allow it like California, New York and many of the southern states. There are then some states that allow or don't allow based on some restrictions. For instance it's generally not allowed in Utah but if you're both over 55 it's fine or like in Arizona it's allowed if you're both incapable of reproducing.
There are even a few states, like Texas, where it's a criminal offense.
You can see what all states follow under where you can or can't but you should really be asking yourself whether you should or shouldn't. And the answer to that is a firm shouldn't.