Is my husband a mahram for my brother’s daughter whom I brought up?
Question

                I have custody of my niece and I am married, is my husband my nieces' mahram and does my niece have to cover in front of my husband in the house, she is 16 teens years of age
Praise be to Allah.
    Allaah has mentioned, in His Book, the men before whom a 
    woman is permitted not to observe hijaab. Allaah says (interpretation of the 
    meaning): 

    “And tell the believing women to lower their gaze (from 
    looking at forbidden things), and protect their private parts (from illegal 
    sexual acts) and not to show off their adornment except only that which is 
    apparent (like both eyes for necessity to see the way, or outer palms of 
    hands or one eye or dress like veil, gloves, headcover, apron), and to draw 
    their veils all over Juyoobihinna (i.e. their bodies, faces, necks and 
    bosoms) and not to reveal their adornment except to their husbands, or their 
    fathers, or their husband’s fathers, or their sons, or their husband’s sons, 
    or their brothers or their brother’s sons, or their sister’s sons, or their 
    (Muslim) women (i.e. their sisters in Islam), or the (female) slaves whom 
    their right hands possess, or old male servants who lack vigour, or small 
    children who have no sense of feminine sex. And let them not stamp their 
    feet so as to reveal what they hide of their adornment. And all of you beg 
    Allaah to forgive you all, O believers, that you may be successful”

    [al-Noor 24:31]

    For more details please see Question no. 
    5538.

    Since the husband of a maternal or paternal aunt is not 
    mentioned in this verse, the ruling is that the general principle of 
    observing hijab in front of him applies in this case, unless this woman (the 
    aunt) had breastfed her brother’s child, in which case her husband would be 
    regarded as a father to her through radaa’ah (breastfeeding), in which case 
    he would be a mahram for her. So if you did not breastfeed this niece, then 
    she has to observe hijaab in front of your husband, in obedience to the 
    command of Allaah, which is best for both parties. That is purer for the 
    hearts and furthest removed from fitnah (temptation). We ask Allaah to 
    reward you and your husband with good for taking care of this girl and 
    bringing her up, and that He will cause that to weigh heavily in the balance 
    of your good deeds.