Latvian

Joke:
Q: How does Latvian climb tree?
A: He use brother's ribs as ladder.

Is funny because is true.

Who the fuck is Latvian?

Latvians are citizens of Latvia, an eastern European country.
I think the joke is that they're poor and starved, so you can use his ribcage as a ladder because his ribs stick out so much.

Oh... Well maybe if you said African or Haitian people would have got the joke.