Should a husband improve his parents' house before completing the construction of his wife's house, especially if his mother is not in need of money and possesses wealth beyond her needs, and if the husband believes that his mother has a greater right to his spending?

If the husband's parents are self-sufficient, then the husband is not obligated to spend on them. However, giving them money is among the greatest acts of proximity [to Allah]. As for the wife's maintenance and housing, it is obligatory even if she is wealthy, and she has priority in spending over the parents. If the marital home is too small and the wife needs more space, and Allah has bestowed abundance upon the husband, then he must first provide suitable housing for his wife, and then he may do as he pleases with his parents' house.