What is the relation in between third oldest person of the family and Y’s mother-in-law and who is third oldest person of the family? 

First try to start blood-relation from the given question. Then try to fix positions of person in the floor of the building. After that you will see in the question, Z lives on odd numbered floor above floor four that means he lives on either floor 5 or 7. If Z will live on floor 7 then the person, who works in SBI lives on floor 5, whereas as it is given in the condition that the one, who works  in SBI, lives on one of the floors below the floor number 4. So that case could not satisfy that Z will live on floor 7. So Z lives on floor 5. Like as you should read all conditions carefully and make floor arrangement. After complete blood relation and Floor arrangement you should find who among the person is older or younger. For that you will clearly see in the blood relation that Z is the father of D and in blood tree no any person is younger than D. So D is the youngest person of the family. Also A and F are the grandfather and grandmother respectively, so they are the oldest person of the family. Similarly fill all conditions and make arrangement