Is it necessary for a woman to observe Purda from her brother-in-law?

Yes, it is absolutely necessary that a woman observes Purda from her brother-in-law. Rasulullah g is reported to have said, “The brother-in-law is the brother of death.” There is a strong fear of Fitna if Purda is not observed. Purda should be observed from the brother-in-law whilst one’s husband is alive and even after death.
Re:- Fataawa Mahmoodiyya pg.97 vol.28