Is it sinful for a woman not to wear niqaab outside her home?

Answer

Yes, it is sinful not wearing a niqaab outside the home. She should always struggle against her desires and make dua to Allah Ta'ala to grant her sufficient courage to don the niqaab. When she is outside without niqaab, she should regret and in her heart seek forgiveness for the sin. If she always remains concerned about this issue, Allah Ta'ala will imbue her with taufeeq (spiritual strength) to don the niqaab and to not care about what others say. A Mu'minah cares of the Displeasure of Allah Ta'ala.