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

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.