If B’s father is A, B is the son of C and D is A’s mother, then how is C related to D?

Answer 1

Either niece of nephew

Answer 2

Since B's father is A and B is the son of C,

then EITHER C=A is the father or C is the mother.

Since D is A's mother, D is the grandmother of B.

if C=A is the father, then D is A's mother.

otherwise D is C's mother in law