In Relation puzzle type, mutual blood relations or other information of more than two persons are mentioned and information about any two is mentioned.
C is A's father's nephew. D is A's cousin but not the brother of C.
How is D related to C ?
C is A's father's nephew means C is the son of A's father's brother
i.e, C is the cousin of A. D is also A's cousin. So, D must be real brother or sister of C.
But D is not brother of C. So, D must be sister of C.