What is the relationship between you and everyone serving in the United States Marine Corps? You are Cousins, unless you are Brothers.
We all have an Uncle Sam. He has children. They are allegedly (occasionally?) misguided.
The son of your uncle is your Cousin.
If you yourself are a Marine, then every other Marine who ever was in the Corps is your Brother. And your Cousin. Out of respect for the combat training my Uncle gave you and your brother/cousin, I will refrain from making the incest joke which logically follows. 🙂
However, the second: Don’t call a Marine your cousin until after he has accepted that this joke is funny coming from you. In any case, they have already made out a check to you, payable in any amount up to and including their very lives, so the proper response when you meet someone and find out they are a Marine, or are (or were) a Sailor, Soldier, or Airman – the response is not “well hi, cousin!” it is “Thanks for your service.”