My SO & I have been together for a year and when we started dating he told me about his little "sister" who's a friend of the family that he's known for years. At first I was a little uncomfortable with the relationship cause they were always texting and talking on the phone but I pushed my concerns away. As the time went on their relationship bothered me more and more. In December she came to visit and while I was at work I seen a picture of her in his bed (fully clothed) & it drove me crazy. So I voiced my concerns and he assured me nothing was going on and his relationship with her is really just like family. And I left it alone. A few months later I had his phone and went into his pictures and found a picture of her topless (covering her boobs with her hands) but that led me to look into their messages where I saw them calling each other sexy & other inappropriate things. So I told him I felt disrespected and didn't want him talking to her anymore. And now I find him texting her and deleting the messages. Gave him a choice either me or her. Do you think  over reacting?