Carl Jones introduced Roger Smith to his friend as his ‘Uncle Roger'. Roger Smith introduced Carl Jones to his friend as his ‘Uncle Carl'. 
"If you are uncles to each other," queried one of the friends, "you must also be nephews to each other." 
"That's correct," replied Carl. 
Since there has been no breach of marriage law, how can you explain such a relationship?

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THE SOLUTION

Two men, A and B marry each others mother. One son was born to each marriage, in this case, Carl Jones and Roger Smith. Carls father is A, and his half brother is B. Also, Rogers father is B, and his half brother is A; therefore, Carl and Roger are both uncle and nephew to each other.