A has two children.
Statement I: C is nephew of D who is not married.
No information about B and G is given hence this statement is not sufficient.
Statement II: F is Son-in-law of B who is mother of E.
No information about G is given hence this statement II is not sufficient.
Statement III: C is grandson of A who is brother-in-law of G.
No information about B is given hence this statement III is not sufficient.
By combining all three statements,
Gender of G not known so all three statements are not sufficient to answer.
Hence, the correct answer is neither of the statements is sufficient to answer the question.