Answer
(Fatwa: 142/D=17/K)
She should tolerate her scolding considering her as mother and should always treat her with love, respect and good morals, in-Sha-Allah by the virtue of good morals and politeness the mother-in-law’s heart will be softened and your sister also shall get its reward. The Quran says: “the good deed and the evil deed cannot be equal. Repel (the evil) with one which is better, then Verily! He, between whom and you there was enmity, (will become) as though He was a close friend.” (Al-Quran, Surah Haa Meem Sajdah 41/34)
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband