How can my sister's brother-in-law comfort him when he quarrels with him?

Fair persuasion. If you are a sister, you can only persuade her fairly. As a sister, first of all, we should understand the reasons for the quarrel between sister and brother-in-law, whether there are any principled problems, and convince sister and brother-in-law from the aspects of marriage difficulty, feasibility, cherish and the influence of husband and wife quarrel on children. It is necessary to educate them to understand and tolerate each other. Only when husband and wife are United and their families are harmonious can they be happy.