Is it a duty upon the husband to ensure that his wife wears hijab and will he be sinful when the wife does not observe it?
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
In the name of Allah, the Most Gracious, the Most Merciful
Allah says in the Qur’an: “You who believe! Ward off yourselves and your families against a Fire (Hell) whose fuel is men and stones.” [66:6] This means that a man has a responsibility for his family members such as his wife and children to ensure they are practising at least the external requirements commanded by Allah.
As for the sincerity of the heart, that is something that only Allah knows. The Prophet also said, “All of you are shepherds and each of you is responsible for his flock. A ruler is a shepherd and he is responsible for those in his care. A man is a shepherd in respect of his family and is responsible for those in his care. The woman is a shepherd in respect of her husband’s house and is responsible for those in her care.” This hadith further highlights the responsibility of a man over his family.
However, if you have tried your best and she still refuses, then Allah will not hold you accountable for that, inshallah.
Only Allah knows best
Written by Maulana Mustafa Umar
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
 Bukhari #2751