How can one reconcile the care of one's parents with pleasing one's wife, given the mother's spending of money on her nephews and nieces and demanding more, and the insufficient contribution of the other siblings?

Thank you for your care of your parents. We advise you not to stop this unless it causes you harm, or restricts your wife, or leads to a shortcoming in her obligatory maintenance. Anything beyond the needs of your indigent parents is not obligatory, but fulfilling their material needs is a form of righteousness towards them. Do not tell your wife everything you do for your parents if it displeases her. Give everyone their due right. A wise man balances pleasing his wife with honoring his parents.