My (28F) boyfriend (33M) has a female "best friend" (25F) that I just don't get.

We've been together for close to 3 years, and I love him very much. He's one of the good ones, and I feel very lucky to have him in my life. <br><br>He's had this friend, Sophie, for what he claims is 12 years. She was a childhood friend that they lost touch with for a few years, but then reconnected when they were all in high school (the same one they currently attend). He also knew she was cheating on her boyfriend at the time, and that led to their breakup. Her boyfriend eventually became one of his friends, along with Sophie. Apparently this has been "Bromance" goal #1 for almost 12 years. <br><br>Sophie is engaged to another man now, but her relationship with my boyfriend remains the same. She still regularly hangs out with the guys, and his favorite thing to post about on social media is the few pictures the four of them take together each week at dinner or other hangouts. If I try to tell him that it makes me feel uncomfortable, he posts about it anyway, as if it's normal. I'm kind of too embarrassed to say anything because they all know each are engaged and know our wedding is upcoming. My boyfriend keeps calling her his "best friend" and talking about how she was there for him when no one else was, which feels weird considering he was there for her affair. <br><br>If I didn't have trust issues already, I would think that he was involved with her. He knows way too much about her sex life for my comfort level, and she knows way too much about his. I know they've never cheated on their partners before, but I don't know that it's impossible. He also knows her fiancé way more than I do, despite knowing him for less time. They will often joke about her being a homewrecker, which I don't get. Why would you make jokes about being a homewrecker if you're not involved with the woman?<br><br>I feel like I'm being gaslit when he says he doesn't like the envious looks, and that he doesn't want to share his friend with no one. I get that they've been friends for a long time, but I also feel like this is an inappropriate friendship, especially since he knows all of her deepest secrets. I don't get why they keep calling her a homewrecker and joke about it. I don't like the closeness that they have. <br><br>I've also noticed that he has never had a "best friend" like this before, and only claims to have had this level of closeness with her. I don't know why he feels the need to prove to everyone that he and Sophie are best friends, or why she allows him to. <br><br>What do you think about this "best friend" situation? Is it normal? What am I overreacting about? Should I ask him to post less pictures of them together?