Is a wife sinful if she seeks divorce due to the immorality of her husband's first wife, whom he took back?

It is not permissible to describe a Muslim as a transgressor without clear proof. The mere fact that the husband took back his first wife does not permit the (second) wife to seek divorce, as long as the husband does not neglect her rights and deals justly between them, based on the Prophet’s (peace be upon him) saying: "Any woman who asks her husband for divorce without any compelling reason, the scent of Paradise will be forbidden to her." However, if it is proven with clear evidence that the first wife is committing immoral acts and your husband is content with that, then this is a defect that permits you to seek divorce from him.