His mother favours his brother’s wife over his wife

Whatever
wrongs your mother does towards you, you have to bear it and be patient.
Indeed, you have to take advantage of her still being alive, to take
care of her, because there is a great reward in that. One of the salaf
wept when one of his parents died, and said, “I used to have two doors
to Paradise; now one of them has closed.” And the Prophet
(peace and blessings of Allaah be upon him) said: “May his nose be rubbed
in the dust (may he be humiliated), whose parents, or one of them, reach
old age during his lifetime, then he is not forgiven.” It is difficult
for parents to control their emotions, and it often happens that parents
are more inclined towards some of their children than others. We have
to put up with that and bear it with patience. Soon enough we – if we
live a long life – will be in the same position.

 Shaykh
Muhammad al-Duwaysh

 If
your mother persists in not praying, then you have to persist in calling
her, with gentleness and kindness, and keep on praying for her to be
guided by Allaah.  

The
one who neglects to pray altogether is a kaafir, but parents are accorded
special treatment even if they are kaafirs, because Allaah says:

“But
if they (both) strive with you to make you join in worship with Me others
that of which you have no knowledge, then obey them not; but behave
with them in the world kindly” [Luqmaan
31:15]