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.