A is the son of B. C is the father of D. A is married to E. A and D are siblings. F is the brother of E. B has only one son. G is the son of A. F is the father of H. I is the daughter of D. F has no daughter. What is the relation of C to E?

Let's trace the family tree step-by-step:"A is the son of B" $\to$ $B\to A[\text{Male}]$."A is married to E" $\to$ $A[\text{Male}]⟺E[\text{Female}]$."A and D are siblings" and "C is the father of D" $\to$ C is the father of both A and D. Since A is the son of B, B must be the mother of A (married to C).Since C is the father of A, and E is married to A, C is the father of E's husband.Therefore, C is the Father-in-law of E.