Praise be to Allah.
The correct view is that a woman may uncover what is above the navel and what is below the knees in front of another woman, whether she is a Muslim or a kaafir. As for the area between the navel and the knees, this is ‘awrah and is to be covered in front of everyone. No woman should see this area of another woman, whether she is Muslim or non-Muslim, related to her or not related; the same applies to the ‘awrah among men (i.e., no man should see the ‘awrah of another man). A woman may see the chest, head, shins, etc., of another woman; similarly a man may see the chest, shins, and head of another man. With regard to the view of some scholars, that a believing woman should not uncover in front of a kaafir woman, this view is superceded by the more correct of the two scholarly views, because at the time of the Prophet (peace and blessings of Allaah be upon him), Jewish women and also idol-worshipping women used to enter upon the wives of the Prophet (peace and blessings of Allaah be upon him) for various reasons; it is not narrated that they used to wear hijaab in front of them, and they are the best and most pious of women.