Is a wife entitled to claim the deferred mahr from her husband who insults and curses her and his sister, demands that she uncover her face in front of his brother and brother-in-law, calls her "ISIS" for not uncovering her face, and allows his sister to curse her and curse Allah, knowing that he interrupts his prayers and Friday prayers, and that she is pregnant and at her family's home?

The dower is an exclusive right of the woman, which she is entitled to in full upon consummation of the marriage. If a deferred dower's due date is specified, she becomes entitled to it upon the arrival of that date. If no due date is mentioned, it becomes due upon separation, whether by death or divorce. Your sister is not entitled to demand the deferred dower before its time, and the actions of her husband or his sister do not justify such a demand.

If your sister's husband is negligent in prayer, he is in grave danger. One who does not uphold the rights of Allah cannot be expected to honor his wife or fear Allah in relation to her.

He has no right to ask her to display her adornment before a non-mahram, and you did well not to obey him in that.

He should be advised and reminded of Allah. If he repents, well and good; otherwise, separation from someone like him is more appropriate. It is recommended for the woman to seek khul' from him due to his abandonment of the rights of Allah the Almighty.

The husband's sister is in a worse state than her brother if she curses the Lord of the Worlds; this is disbelief by the consensus of scholars. It is obligatory to advise her and warn her of Allah. If she does not repent, then her matter should be reported to the responsible authorities to deter her from such actions.