All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
Since that man is your brother-in-law, you should respond to his abuse with kindness, as this greatly influences any rational person to stop abuse and injustice; Allaah says (what means): {And not equal are the good deed and the bad. Repel [evil] by that [deed] which is better; and thereupon the one whom between you and him is enmity [will become] as though he was a devoted friend.} [Quran 41:34]
By pardoning and being patient, your reward will be greater; Allaah says (what means): {…but whoever pardons and makes reconciliation - his reward is [due] from Allaah.} [Quran 42:40] Allaah also says (what means): {And whoever is patient and forgives - indeed, that is of the matters [requiring] determination.} [Quran 42:43] Allaah further says (what means): {But if you pardon and overlook and forgive - then indeed, Allaah is Forgiving and Merciful.} [Quran 64:14]
Besides, the Prophet said to ‘Uqbah ibn ‘Aamir, “Maintain ties of kinship with those who have cut you off, give alms to those who refrained from giving you, and pardon those who have wronged you.” [Ahmad - al-Albaani graded it saheeh (sound) in as-Silsilah as-Saheehah]
As regards supplicating against the wrongdoer, then this is permissible; the evidence for this is the saying of Allaah (which means): {Allaah does not like the public mention of evil except by one who has been wronged. And ever is Allaah Hearing and Knowing.} [Quran 4:148]
Ibn Katheer said in his Tafseer, “Ibn Abbaas said about the interpretation of the verse: Allaah does not like that one supplicates against anyone unless he is oppressed, Allaah permitted him to supplicate against the one who wronged him, that is His saying: {except by one who has been wronged}, but if he is patient then it is better for him.” For more benefit, please refer to fatwa 130288.
Also, there is nothing wrong in supplicating with death, as it is confirmed that the Prophet supplicated for the safety (salvation) of the oppressed and supplicated against those who wronged them. Abu Hurayrah narrated: "When the Prophet raised his head from rukoo’ (bowing) in the last rak’ah (unit of prayer), he said, 'O Allaah, save ‘Ayyaash ibn Abi Rabee’ah, O Allaah, save Salamah ibn Hishaam, O Allaah, save Al-Waleed ibn Al-Waleed, O Allaah, save the oppressed in Makkah. O Allaah, tighten Your grip on Mudhar and send them years of famine like the years of famine of Yoosuf (Joseph ).'" [Al-Bukhari and Muslim]
Badr ad-Deen al-'Ayni said in Sharh al-Bukhari, “This hadeeth is evidence of the permissibility of supplicating against the oppressor with death and supplicating for the Muslims with salvation.”
Therefore, there is nothing wrong in supplicating against your brother in-law with punishment according to his injustice; as Sa’eed ibn Zayd supplicated against the woman who wronged him. Muslim narrated in his Saheeh from ‘Urwah from his father that Arwa bint Uways claimed that Sa’eed ibn Zayd had usurped part of her land, so she complained against him to Marwaan ibn al-Hakam. So Sa’eed said, “How would I take part of her land after what I heard from the Prophet ?” Marwaan ibn al-Hakam asked him, “What did you hear from the Prophet ?” He said, “I heard the Prophet say, 'Whoever usurps the land of somebody unjustly, his neck will be encircled with it down the seven earths (on the Day of Resurrection).’” Marwaan said to him, “I will not ask you for further evidence.” He said, “Oh Allaah, if she is lying, then take away her eyesight and kill her in her land.” He said, “She did not die until she became blind, and while she was walking in her land, she fell inside a pit and died.”
However, it is more appropriate not to supplicate against him with death, especially if he has children, as his death will make your sister a widow and her children will become orphans.
As regards supplicating against him to be punished with a punishment that is more than his injustice to you, then this is also an injustice; Sharh Mukhtasar Khaleel authored by al-Kharashi reads:
“It is permissible to supplicate against the oppressor (unjust ruler) that he be deposed whether he oppressed him (the one who supplicates) or oppressed others. However, it is more appropriate not to supplicate against someone whose injustice is not spread; if his injustice is widespread, then it is more appropriate for the oppressed to supplicate against the oppressor. Nonetheless, he is prevented from supplicating that his children or his family (wife) die or that he commits a sin; because wishing a sin (for others) is a sin in itself, or to supplicate against him with a harm that will affect him with more than what he deserves. There are two opinions in regard to supplicating against him with a bad end to his life; the most preponderant opinion, as stated by Ibn Naaji and others, is that this is prohibited, contrary to the view of al-Burzuli.”
Also, al-Fawaakih ad-Dawaani ala Risaalat Ibn Abu Zayd al-Qayrawaani reads:
“Al-Qaraafi said, 'There are situations in regard to supplicating against the oppressor: firstly, to supplicate against him to be deposed so that his injustice will be stopped, and this is good, or secondly, to supplicate that his children die as well as his wife and others who are related to him but who did not do any wrong to the person, and this is forbidden due to harming innocent people. Thirdly, it is to supplicate against the oppressor to commit a sin, like him being afflicted by drinking alcohol or backbiting or accusing innocent people of adultery, then this is also forbidden because wishing a sin for others is also a sin. Fourthly, supplicating against the oppressor to be harmed in his body more than what he deserves as a punishment; this is also an inappropriate act, as Allaah says (what means): {So whoever has assaulted you, then assault him in the same way that he has assaulted you.} [Quran 2:194]."
Allaah knows best.