Is a Muslim woman sinning if she takes off Niqaab after wearing it because she feels that there is a greater benefit in taking it off because of the purpose of giving Da'wah to her non-Muslim family?
Note: that I live in Australia and I feel that Niqaab is a Sunnah (Albani report on Niqaab) and Allaah Knows best. My family really dislike me wearing the Niqaab and this puts a barrier in giving Da'wa to them.
Isn't a principle in Fiqh that you should leave off a Sunnah act for the greater benefit of giving Da'wah? Are we not in these lands (Australia) in order to give Da'wah?
so my point is, is it better to leave off this Sunnah of wearing Niqaab in order to give better and more proper Da'wa to my non-Muslim family members?
I've talked to my husband on this matter and he is not sure if I should take that step in taking it off because he feels I will be getting less in my religion, but I feel that there is a greater good in taking it off. I'm not sure if its proper of me to take it off, please advise me according to the Quran and Sunnah.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

According to the most preponderant opinion of the scholars a pubescent woman is obliged to cover her face, especially as we live in a corrupt era where temptation is widespread. This is our view but there are different opinions among the scholars on this matter and it is an issue of Ijtihaad (a juristic opinion on matters which are not specified in the Quran or the Sunnah).

In general, if a woman is content and pleased with an opinion that is contrary to the opinion of her husband who thinks that his opinion is closer to the truth, then she is not obliged to abide by the opinion of her husband. Anyway, in this issue, she should cover her face, as this opinion is closer to the truth and more protective of a woman's dignity.

However, if you are sure that by unveiling your face, you would achieve a greater benefit or repel a greater harm, then the spouses should not differ about the permissibility of uncovering the face in order to achieve that benefit or repel that harm, especially that covering the face is a matter of difference of opinion among the scholars.

In this issue one should resort to trustworthy scholars in your country, as they are more aware about the situation there and it is hoped that they will give you the appropriate ruling. 

Allaah Knows best.