Is it permissible for a husband to oblige his Christian wife to wear Hijaab? Does he bear a sin for not enjoining her to wear it?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

Allowing one's wife to expose her beauty and adornment is not the attitude of chaste people. Rather, it is the attitude of the people of immorality and depravity! Hence, a Muslim husband should guard his wife and forbid her from exposing her beauty and adornment, even if she is not a Muslim but belongs to the People of the Book. He is entitled to, and is even obliged to, forbid his wife from assuming the appearance of immoral women.

If she refuses, he should abandon her in bed, i.e. not having sexual intercourse with her. If she insists on her disobedience, he should  gently strike her. The striking should not be severe and should not leave any scars or marks on the body or cause bone fractures. This is because it is a disciplinary striking and not a punishment. If she persistently insisted on her disobedience and exposed her beauty and adornment, then a believing husband should not keep such a wife who exposes her beauty and adornment before men, imperiling his reputation and her honor.

Allaah Knows best.