When it comes to non-Muslim women looking at Muslim women, there is a narration in our sources from Imam Ja'far Sadiq (a.s.) who said that: "It is not appropriate for Muslim women to remove their Hijab (covering) in front of Jewish or Christian women because they tell their husbands about their (Muslim women's) physical features." This narration consists of certain points which we will explain briefly:
1. It is inferred from "it is not appropriate for" that removing hijab in front of Jewish and Christian women is abominable. That is to say, it is undesirable for Muslim women to uncover themselves in front of non-Muslim women. Prominent Shia jurists have also issued verdicts as to the abominableness of removing hijab in front of such women.
2. The terms 'removing hijab' refer to abominableness of revealing all parts of the body except the face and hands (to the wrists) because there is no problem in exposing these two of the body even to men. Of course, it is haram for women to look at the private parts (genital area) of a woman irrespective of whether she is Muslim or non-Muslim. Therefore, the narration refers to all parts of a woman's body except her private part and her face and hands.
3. It is inferred from the reasoning in the narration that there is no difference between Muslim and non-Muslim women in removing hijab. Hence, it is necessary for a Muslim woman to cover her body in front of any women who is likely to tell her husband about the physical features of a Muslim woman.
4. The last part of the narration argues that a Muslim woman should not expose her body to women from the People of the Book because they tell their husbands about her physical features. This argument implies that there is not any difference between husband or someone else in telling them about the physical features of a Muslim woman. That is to say, the same ruling applies to them, if they manage to speak about her physical features to someone other than their husbands.
The question which arises here and which you have also raised in your message is: If they do not reveal the physical features of a Muslim woman to their husband, is it permissible for Muslim women to remove their hijab (covering) in front of them?
If a Muslim woman becomes certain that they will not do as such and, like Muslim women, they will not tell their husband's or others about her physical features, it is not necessary for her to cover herself from them. In this case, as some jurisprudents have stated in their fatwa, one can consider it (removing hijab) as permissible.
 - Kulayni, Muhammad bin Ya'qub, Al-Kafi, vol.5, chapter al-Tasattur (covering), pg. 518.
 - Najafi, Jawaher al-Kalam fi Sharh Sharaye' al-Islam, vol.29, pg. 63; Mawsu'at al-Imam al-Khoei (Khoei Encyclopedia), vol.32, issue No.28, "Yajuz li-kolle min al-rajole wal-mar'at-e al-Nazaru elaa ma 'Ada al-'Awrah… pg.28
 - Al-Hakim, Sayyid Mohsen, Mustamsak al-'Urwatul Wuthqa, vol..14, issue No.28, pg. 22