Should I confront my mother?

My whole entire life I have been lucky enough to have my parents married. However over the past couple of years I have noticed that things are not going well. I would ask my parents how they are feeing with one and another, and they both would be like “he/she does communicate with me” I assumed this would mean a divorce (which is fine just want them to be happy). However, this weekend I came home to go out to dinner with my family. We went out and it was fun they got along like they used to. However when we got home my mom had her phone open texting some guy flirtatous texts and basically saying things like “my soon to be ex is leaving for a week can't wait see you” (dad is going away for a week). This shouldn't bother me but it does. I don't mind the divorce but I just feel like this is the wrong way to go about it. My younger sister has no idea what's going on as I am 3 years older than her. Do I talk to my mom, sister, or dad? I just feel wrong