I think my father is cheating in my mother.
Oh no this is very serious. Are you sure about that?
Yes, I know it for sure.
If you have the proof of it, I think that you should tell her. It would be wrong to withhold that information from her, it will hurt her even worse if she finds out that you knew.
My mother has been cheating on him and I know they are getting a divorce. My brother and father deserve this.
If you know someone is cheating you should say something. It is bad to let people get away with cheating behavior.