Is the wife considered sinful if she takes from her husband's money without his knowledge to cover the expenses of their son's marriage, which she had committed to, especially since her husband refused to pay more money?

A father must help his son marry [and become chaste]. There should be moderation in the expenses of marriage, so as not to reach the point of extravagance and wastefulness, which are forbidden by Islamic law. A wife is not permitted to spend from her husband's money without his permission, unless he withholds the obligatory maintenance. If a wife spends from her husband's money for the son's marriage without his permission, and the amount spent by the husband was sufficient, then she must repent and seek forgiveness, and seek absolution from her husband by informing him and asking for his pardon, unless she predominantly believes that informing him would cause harm to their relationship. In that case, she is not obliged to inform him, and she should return the money from her own funds if available. Otherwise, she should repent, seek forgiveness, and be kind to her husband.