Not only are Christian nuns also wearing a hijab, but the Bible says all Christian women are supposed to as well.
Corinthians 11: 6
{... And every woman who prays or prophesies with her head uncovered dishonors her head, for it is just as if her head were shaved.
6
If a woman does not cover her head,let her hair be cut off. And if it is shameful for a woman to have her hair cut or shaved off, she should cover her head.
7
A man ought not to cover his head, since he is the image and glory of God; but the woman is the glory of man.… }
Modesty is essentially personal belief, which essentially is the same as religion.
So not allowing a hijab is no different than demanding women be naked in public.