When you find out that your mother is cheating on you, you should first consider your own feelings and needs. If you find it unbearable, you can consider talking to your mother or brother alone to express your thoughts and feelings and seek solutions. There were still some factors to consider before deciding whether to tell her brother. For example, whether she thought that her brother would feel the same emotional distress or whether it would negatively affect her brother's postgraduate entrance examination. If she decided to tell her brother, she should choose a suitable time and place and respect her brother's privacy and feelings. No matter what you decide to do, you should remember your own feelings and needs.