Is the wife entitled to prevent her husband from sending money to his family, who claim to be in need, while she knows they are not, and that his brother, who requests money for his studies, is morally corrupt and capable of working?

You are not permitted to be an obstacle to your husband in his devotion to his parents and maintaining ties with his kin, especially if his father is unemployed. The responsibility of devotion to parents is shared among all children, and the most important aspect of devotion is financial support. If your husband's siblings fall short in their duties towards their parents, you should be a helper to your husband in his devotion to them. The misconduct of one sibling or the lack of assistance from others should not prevent your husband from supporting his parents and siblings.