Hi All,

Legally, Mexican codes define and accept three kinds of relationships: 
consanguinity (related by blood) 
affinity (acquired by marriage) 
civil (by adoption) 

I found a good link that explains this.  Please see it below: 
http://www.dropby.com/indexLF.html?content=/Genealogia/parentesco.html 

On the real world, however, Guillermo is right.  Any kid from one of your cousins is your nephew/niece and he can call you tío/tía.

Victor