Am I obligated to give my mother her full monthly allowance, knowing that she will distribute it among my siblings and their children, which exposes her to malnutrition, especially since I give her her allowance weekly?

You are not obligated to pay your mother the full monthly allowance, especially if you know that she does not spend it on herself. Rather, pay it in a way that serves her best interest. Be good to your siblings by honoring them and giving them charity, especially if that pleases your mother. Strive to advise your mother to take care of herself and not to give charity except from what is in excess of her needs.