r/u_throwawayacnt2423

My wife talks shit about my family to her sister but in front of me she speaks well and behave well with my family. I am not sure what is the issue here.
I found out by going through WhatsApp chat between my wife and her sister and from then onwards I secretly read their chat every week and every week I find something like this.
She doesn’t have any idea that I know what they talk about.
I don’t feel good at all. What to do here?