Will the spouses be held accountable for their feelings towards the mother, due to her repeated requests for money for her son and the husband's waiver of his inheritance to his brother? And is the mother entitled to ask for any money for her youngest son?

If a weakening of love for the mother does not result in a dereliction of her rights or ill-treatment of her, then there is no sin, for love and hatred are matters that Allah Almighty places in the heart. There is no blame on the husband for refusing to give his mother money for furnishing his brother's apartment or similar things if the mother is self-sufficient, for she has no right to her son's money. However, it is preferable for the husband not to withhold anything from his mother that does not harm him, for the mother's right is great, and honoring her is among the best deeds.