Sister-in-law has a bad relationship with her parents. What should she do as a sister?

The relationship between mother-in-law and daughter-in-law has been a difficult problem since ancient times. As a sister, we should communicate well in the middle, be a peacemaker, and speak well of our mother in front of our brothers and sisters-in-law. Instead, I should say more good things about my brother and sister-in-law in front of my mother and adjust them in the middle. Slowly, their relationship will get better and better.