Praise be to Allah.
Your keenness to be modest in dress in front of kaafir women is something to be commended and appreciated, especially at a time when there is so little modesty and gheerah (protective jealousy – of men over their womenfolk) is so weak.
With regard to the issue of kaafir women seeing what ordinarily appears of the Muslim woman, this is an issue concerning which the Muslim scholars of the past and present have differed. Under Question no. 2198 you will find a summary of what the scholars have said and the evidence for each opinion. You will notice that those who say that it is permissible for a kaafir woman to see a Muslim woman without hijaab have more than one piece of evidence to support their view.
But it is worth pointing out the following:
1 – If a Muslim woman has saheeh evidence concerning some matter of sharee’ah, and the evidence is clear and the meaning is apparent, then she should follow it; it is not essential to have two or more pieces of evidence for every issue.
2 – If the kaafir woman is not trustworthy and there is the fear that she may describe the Muslim woman to her husband or say something that will harm her honour, then the Muslim woman has to beware of her by wearing hijaab, not mixing with her and so on.
3 – The view that Muslim women should wear hijaab in front of kaafir women is a valid opinion and is correct to some extent. Many of the salaf (first generations of Islam) expressed this view. So if a Muslim woman wants to be on the safe side regarding her religion and follow this opinion, she may do that. It is hoped that her caution and her keenness to avoid something doubtful will bring good results. But she should not denounce those who differ from her in that, so long as the matter is subject to ijtihaad and differences of scholarly opinion, especially when there is no fear of fitnah from the kaafir women.
4 – Women should not go too far in uncovering in front of their mahrams or Muslim women, let alone in front of kaafir women. Rather they should not show any more than that which ordinarily appears, namely the hands, calves, neck and hair. Modesty is a branch of faith. You will find more details and further explanation under Question no. 6596. And Allaah knows best.
May Allaah bless our Prophet Muhammad.