Is it possible for a Muslim woman to take of her scarf (Hijab) in front of Christian women?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The ‘Awrah (those parts which should be covered) of a Muslim woman in front of fellow Muslim women is between the navel and the knee. But the ‘Awrah for a Muslim woman in the presence of a non-Muslim woman is the whole body since Allah Says (what means): {And tell the believing women to reduce [some] of their vision and guard their private parts and not expose their adornment except that which [necessarily] appears thereof and to wrap [a portion of] their headcovers over their chests and not expose their adornment except to their husbands, their fathers, their husbands' fathers, their sons, their husbands' sons, their brothers, their brothers' sons, their sisters' sons, their women, that which their right hands possess, or those male attendants having no physical desire, or children who are not yet aware of the private aspects of women.} [Quran 24: 31]

The words "their women" in the above verse mean fellow Muslim women. Had it been permissible for non-Muslim women to look at Muslim women, then it would not have been mentioned in this verse.

It is proved from ‘Umar, may Allaah pleased with him, that he prohibited the women of the People of the Book to enter saunas with Muslim women. The ‘Awrah of a Muslim woman in-front of her (Mahaarim) unmarriageable relatives is no more than her face, head, hands and feet, so it is forbidden for her to show her chest and breast in front of her Mahaarim. All the mentioned rulings apply in normal situations but if she is compelled to disclose her body in front of a non-Muslim for treatment and the like, then she can do so. No doubt disclosing her body in front of a non-Muslim woman is easier than disclosing it in front of non-Mahram Muslim men.

Allah knows best.