What is the ruling regarding a wife who agrees with her husband to reveal the past, then it becomes clear that she concealed information and lied after swearing to tell the truth?

Lying between spouses is permissible if it leads to a legitimate benefit, such as maintaining the relationship. However, it is better to use allusions (al-ma'areedh). It is not permissible for either spouse to disclose their past sins to the other; rather, repentance and self-concealment are obligatory. If a wife swears that she was truthful when she was lying, this is a "yameen ghamoos" (a false oath taken intentionally), from which repentance is obligatory.