There is a Chilean inside ‘joke’ or more an excuse used for when someone is caught being a little (if that is possible) unfaithful with someone else.
Scene 1
Picture a guy being overly affectionate with a woman (or girl) that is not his girlfriend.
Then imagine that his girlfriend sees him doing this and rushes up to him and says (or more likely yells),
“What are you doing?”
How do you think he might respond?
“It doesn’t mean a thing”…. No
“It’s not what it seems”…. No
“She’s my cousin”…. Yes!
The guy is most likely to deny that he has done anything wrong and comes out with the expression, “Es mi prima.” (She’s my cousin).
Why would he say that?
Well, family members are often quite affectionate towards each other in Chile, with large helpings of cuddles, hugs and other types of cariños (displays of affection) being common.
Also, families tend to be large in Chile and extended families with all the uncles, aunts and cousins lumped in there can be HUGE!
Just imagine, my wife has 17 or so tios (uncles/aunts) not including their partners. Then if you add the cousins (up to 4 per family)… we’re looking at 50 or more relatives.
So even though we have known each other for over a decade, every now and then I get introduced to a new cousin at some family get together (and no, not under the suspicious circumstance that is mentioned in this post). Really, this person is her cousin (male or female) that I have never met.
The same probably happens to many couples in Chile where they sometimes never meet, or personally know all of the members of the extended family. This is why the expression would work so well… if played properly.
HOWEVER, it’s can be a little harder for the unfaithful one to use this expression once they’re married since by then they usually know the rest of their partner’s family.
Sorry, foreigners can’t use it!
Funny thing is that as a foreigner in Chile I would never get away with using this expression since my whole family is thousands of miles away! Not they I WOULD ever need to say it. 
So next time your partner says this… beware!
Unless of course there is another family member nearby to confirm…
(however they could also be an accomplice to the crime). 
Have you met any cousins of late?
{ 2 comments… read them below or add one }
Yes.. there are still cousins (and other relatives) I’ve never met after 20 years of extended trips to Chile, and now into the tenth month of living here permanently.
All the best…
Let’s just hope they really ARE relatives.