1, you have to tell dad not to worry, you have to trust mom, she won't cheat. Or find a way to make her social interaction more transparent in her mother's behavior. For example, I don't need to chat with electronic devices, dance and play mahjong, and chat with men. I usually call my father to tell him what I am doing and what I am eating to meet his safety needs.

Like an adult, or a mature person, let your mother go to a place where there are few people to talk. You must not lose your temper and don't "threaten" her for the time being, because in that case, what you lack is self-confidence, and your mother will think that you are still young and ignorant, so your "maintenance" may only make your mother "uncomfortable". But you are so kind, not just trying to save the superficial phenomenon of mom and dad's marriage ~ I think you should also take care of the overall situation and leave some face and dignity for your mother.

2. First of all, you should make sure that your mother really has an affair. Maybe her mother was infatuated for a moment. You can, find a quiet time with a better atmosphere, and mother and daughter can sit down and talk about the recent changes of mother and listen to what she has to say. Just say your doubts directly (don't say that you found out about her affair with that person), just say how your family feels, what you want this family to be like, what you are worried about, how your family came over in the most difficult time, and how much you hope that your future marriage life will be as beautiful as theirs.

In fact, some things we do as children don't need to restrict or hinder some things of adults, and sometimes we can't change anything. Maybe today, if you stop adult things, it will lead to serious things. Therefore, as children, we can only let nature take its course, which can be implied in some places, and sometimes it is a wise choice to play dumb.