A is the brother of C. B is the only sister of D who is the maternal uncle of C. If E is the father of C, then how is A related to B?

Son

Note the point: 

i) B is the only sister of D and D is the maternal uncle of C, it means C is the daughter/son of B. 

ii) Further, E is father of C, therefore E is husband of B only.

Now, ATQ,

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Please note:

  l   ⇒ Vertical line shown "generation gap".

 =  ⇒ Double lines shown "relation of spouse" 

—  ⇒ Horizontal line shown "relation in same generation other than spouse".

     ⇒ Rectangular box shown " Male".

     ⇒ Circular box shown " Female".

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