Hiding text messages!!

My so is friends with his good guy friend little sister and they text and talk on the phone but I decided to look in his phone because I am getting insecure and there was only one text from her? And a few weeks ago she sent a picture asking if she looked pretty. I seen it because it popped up when I had his phone!! Am I wrong for getting insecure I brought it up and said I want to tell her to please stay out of my relationship and he won't let me. Mind you she has a bf and it's my SO friend that she is dating! My SO told me she said to him a while ago that she is mad we are working things out and then now is saying do what makes him happy!