Would the wife's departure from her mother-in-law's house, where she lives with her husband and children, be considered an injustice to the mother-in-law or to the children, given that the mother-in-law owns two houses and refuses to sell one for expansion, and the husband refuses to rent for fear of angering his mother?

There is no objection to the husband moving to another residence, and there is no injustice or disobedience to his mother in that. He must continue to honor her and be good to her, and beware of mistreating or neglecting her rights. The wife is advised to help her husband honor his mother, to be patient with her, to think well of her, and to overlook her shortcomings; for countering evil with good brings affection and repels the whispers of Satan, as Allah Almighty said: "Nor can goodness and evil be equal. Repel (evil) with what is better: then will he between whom and thee was hatred become as it were thy friend and intimate!"