Is a son considered sinful if he does not listen to his mother who mistreats his wife, and how can one reconcile between the mother and the wife under these circumstances?

Obedience to the mother is obligatory through showing kindness to her and associating with her in a good manner, but not at the expense of wronging the wife or abusing her. One should deal wisely with both the mother and the wife so that the wife is not wronged and the mother is not disobeyed. This is achieved by pacifying the mother and pleasing her in a good way without diminishing the wife's rights, while advising the mother with politeness and gentleness and reminding her of the Almighty's saying: "So never say to them [even] 'Uff!' nor repulse them, but speak to them a noble word. And lower to them the wing of humility out of mercy and say, 'My Lord, have mercy upon them as they brought me up [when I was] small.'" The wife is also advised to show kindness to her husband's mother, and the mother is advised to fear Allah and not seek to corrupt the relationship between her son and his wife, for corrupting relationships and slandering are among the grave sins.