My sister cheated on me and wanted to divorce her brother-in-law. What should I do?

It's your sister's fault that she cheated. No wonder your brother-in-law, every man can't stand his woman wearing a hat for himself. Your brother-in-law's request is also reasonable. All you have to do is talk to your sister, let her know her mistake and get your brother-in-law's forgiveness. Otherwise you can't help it. They must be husband and wife. If you think your brother-in-law is good and like his divorce, you can marry him!