There is a family of six persons A, B, C, D, E and F. They are Lawyer, Doctor, Teacher, Salesman, Engineer and Accountant There are two married couples in the family. D, the Salesman is married to the Lady Teacher. The Doctor is married to the Lawyer. F, the Accountant is the son of B and brother of E. C, the Lawyer is the daughter-in-law of A. E is the unmarried Engineer. A is the grandmother of F. How is E related to F?

C is the daughter-in-law of A who is the grandmother of F means C is the mother of F. But F is the son of B. 

So. B is C's husband. But C, the lawyer is married to the Doctor So. B is the Doctor. F, the Accountant  will be the son of B and C. E is the unmarried Engineer 

So. the other mamed couple can be that of grandmother of F i.e, A and D.  But D, the Salesman, is married to the Lady Teacher. 

So. D, the Salesman, is the grandfather of F. father of B and the husband of A, the Lady Teacher. 

Clearly, from the given data, the relation between E and F cannot be determined.