Is a woman required to wear the hijab in front of her Christian maternal uncle?

A woman is permitted to uncover her hijab in front of her Christian maternal uncle, because he is a mahram to her, and there is no evidence that Muslim women must observe hijab in front of their disbelieving relatives. However, the mahram must be trustworthy. If he is not trustworthy, and there is a concern that he might describe her to others or be tempted by her, then she must observe hijab from him, whether he is Muslim or disbeliever.

As for a disbelieving woman looking at a Muslim woman, the preponderant opinion is that it is permissible. The prohibition applies if there is a concern that she might describe the Muslim woman to other non-mahram men, whether the woman describing her is a disbeliever or a Muslim.