Answer
Rasulullah (Sallallaahu Alayhi Wasallam) said, ‘The brother-in-law is death
(to the sister-in-law).
In view of the Hadith mentioned, you should exercise strict Hijaab with your
sister-in-law. Do not look at them or talk unnecessarily.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai