“The only son of my grandfather” = Rinki’s father.
“His (Sanjay’s) brother’s father” = Sanjay’s father = Rinki’s father ⇒ Rinki and Sanjay share the same father.
Hence Rinki is Sanjay’s sister.
C’s gender is not given. So “C is the brother of A” cannot be asserted; C could be sister.
The other statements can be true under usual family assumptions.