Answered by Sayyidi al-Ḥabīb `Umar bin Hafīẓ (may Allāh preserve him and benefit us by him).
My mother is not a Muslim and she forbids me from wearing the qamīs or thawb, although I am in a place where wearing it is effective in calling people to Islam. Should I obey her or continue to wear it?
It is sunnah to wear the qamīs if one wears it with the intention of following the Prophet (may Allāh bless him and grant him peace). However, we have been commanded to treat our parents well and this dictates that you do not wear it in front of your mother. It would not be considered disobedience to wear it to the Friday prayer or in gatherings of knowledge. At times other than these you need to decide whether the benefit of wearing it outweighs the negative effect it has on your mother or not.