All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad (sallallaahu 'alayhi wa sallam) is His slave and Messenger.
May Allah reward you with goodness for your concern for modesty and wearing the niqab (veil). We ask Allah, the Exalted, to assist you, grant you success, and accept it from you.
Furthermore, be keen on being dutiful to your mother, showing your concern to obey her, and doing what pleases her in matters that do not involve disobedience to the Creator.
As for the issue of covering the face and whether it is considered part of a woman’s ‘awrah (parts of the body that must be covered) or not; some scholars are of the view that covering it is obligatory, while others hold that it is not obligatory. What we consider more correct is that covering the face is obligatory. Therefore, it is not permissible for you to obey your mother by uncovering your face in front of your sister’s husband, as he is a non-mahram (marriageable man) to you. It is authentically reported from the Prophet (sallallaahu 'alayhi wa sallam) that he (sallallaahu 'alayhi wa sallam) said: “There is no obedience in disobedience to Allah; obedience is only in what is right.”
Even according to the view that covering the face is recommended rather than obligatory, obedience to one’s mother is not required in every situation; rather, it is required in matters where she has a sound and legitimate interest. We do not see any valid benefit for the mother in her daughter uncovering her face in front of non-mahram men.
We advise you to be gentle in dealing with your mother regarding this matter, to discuss it with her kindly, and to seek the help of people of knowledge and virtue in addressing her. Lastly, strive to earn her kindness and pleasure if she becomes upset with you.
Allah Knows best.