What should a wife do about her husband whom she observed peeping at his daughter through the bathroom window, and he admits to her that this is not the first time he has spoken about his daughters during intercourse?

If what was mentioned about the husband is true, then this is evidence of weak faith, lack of shame, absence of fear of Allah, and a perversion of his innate disposition (fitra). He must be advised and reminded of Allah Almighty. If he repents and rectifies his conduct, then praise be to Allah. Otherwise, he should not be allowed to be alone with his daughters, and he must be treated as a stranger, with the adult daughters veiling themselves from him. If the husband persists in his actions and the mother fears for her daughters, she is permitted to seek a divorce from him. Hanbali jurists have stipulated that the husband's transgression in the right of Allah is one of the justifications for seeking divorce or khul'. However, consideration must be given to what happens after the divorce: will she take the daughters with her, or will they remain with him? If the latter is a possibility, then it is better for them to stay with him until a ruling is issued between them or the mother finds another solution. For your information, custody is not granted to a corrupt father from whom there is fear for his daughters if they are in his custody.