Ask your sister to have patience and keep on behaving with her husband in good manner. If he does second marriage, then too she should live with him and ask him not to give her talaq. To get her husband become kind, ask your sister to take 11 pieces of black peppers after Isha salah, and recite Ya Latif Ya Wadood (يا لطيف يا ودود) 1100 times with durood 11-11 times before and after, and while reciting she should think of her husband to be kind. After completing this act, she should blow it on the black peppers and put it into flaming fire and supplicate Almighty Allah. In-sha-Allah, her husband would turn kind. This practice should be done for forty days. (Bahishti Zewar) In such conditions it is better for a wife to turn towards Almighty Allah and behave with husband in good manner. Leaving all these things and resorting to jurisdiction and behaving husband tightly bring nothing but loss. They do nothing but lengthen the dispute and increase its gravity.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband