Praise be to Allah.
The Muslim should always give preference to his mother, because it says in a hadeeth that a man said to the Prophet (peace and blessings of Allaah be upon him): “Who among the people is most deserving of my good company?” He said, “Your mother.” He asked, “Then who?” He said, “Your mother.” He asked, “Then who?” He said, “Your mother.” …
(Narrated by al-Bukhaari, 5514; Muslim, 4621).
But the wife takes precedence over the mother in one case, and that is the matter of spending. If the husband cannot afford to spend on both his wife and his mother because he is poor, then in this case he should put his wife before his mother. The Muslim has to give what is due to each person who has a right over him, and he has to help the one who is oppressed. If his mother mistreats his wife, he has to put a stop to it, in a kind and fair manner.
And Allaah knows best.