Answer
(Fatwa: 1793/1267/L=1432)
If the uncle of your wife has that much money that he may travel to and fro Makkah for hajj and can arrange the instalment of debt as well as the expenses of family then hajj is fardh on him. If he has that much money that may suffice for the hajj expenses of both (the husband and wife) then they should perform their hajj. The obligation of hajj shall be discharged from both of them. Ulama have written that if a poor man performs hajj then it will not be fardh on him to perform hajj again after he turns rich later.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband