If a person's wife/sister/mother doesn't observe purdah, what is the shariah ruling

One should try to educate the wife in obseving purdah, explain the benefits and let her talk to other Muslimah who wear purdah, and how they feel about it. Make dua for her and help her out in trying a purdah on at times. Continuously encourage her, but do not be vigorous as this will only make matters worse.

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai