Answer:
(Fatwa: 1421/866/H=1432)
The younger son is right in saying. It is not lawful for father-in-law to stay with his daughter-in-law alone that too for the whole day. He (father) should stay with his wife.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband
