What is the relevence between the 3 stories in Babel ( In Detail ) ?

Answer 1

Yeah, the guy above me got it right.

I will just repeat what he said in my own way to help you better understand. (In case you are confused.)

3 stories -
1) Moroccan boys
2) Tourists (Brad and Kate's characters)
3) Japanese girl

One of the tourists is the primary victim to a crime caused by one of the 2 boys, whose Father got their weapon through the Japanese girl's Father.

The movie essentially shows all the cause and effect of people's actions, the same as say a drug dealer who sells some coke on the street.
The coke dealer indirectly becomes a murderer and adds to the exsiting problems of the world in so many ways, with just that one sale.

So many issues were recognized in this movie, it is difficult to cover them all, but that was pretty much the message.

Answer 2

The 2 kids that were watched by the mexican maid are the children of Brad Pitt & Kate Blanchett character's. The gun that was used by the moroccan kids that shot the american(Blanchett), used to belong to the japanese guy(father of the japanese girl).