Answer
The father-in-law is a Mahram (prohibited in marriage) to his
daughter-in-law.
In principle, she does not have to observe Hijaab (Purdah) between her and
her father-in-law. However, if they both are young, they should display
modesty between them especially in these times of corruption and immorality
when incest, rape, etc. are common.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai
FATWA DEPT.