A is the son of B, whose mother is C. D is the husband of E the grand-daughter of C. If F is the wife of A, how is F's father-in-law related to D's daughter?

From the given information we can draw the following family tree 

We can observe that A and F are husband and wife. A is the son of B. C is mother of B. C is grandmother of A and E. Lastly, D is husband of E. 

We need to find the relation between F's father in law and D's daughter. 

F's father in law can be be B or husband of B, ( we can't say exactly as we don't know the gender of B) 

E and D's have C as their grand mother and B and/or his husband as Uncle. 

Hence E and D's daughter will have F's father in law as her grandfather.