Boyfriend's best friend is a girl😕

So my boyfriend has this really good  friend who is a girl and they used to sleep round eachothers houses and they still always go to eachothers houses and text all the time and she put her name as 'princess😘😍' on his phone and they send tons of kisses to eachother over text. He gets along so well with her family and her parents have always wanted them two to date. He says they are just like brother and sister and that nothing has or ever will happen with them but idk it still bothers me and he doesn't get why it bothers me. If it was the other way round he would hate it! My ex boyfriend was really good friends with my friend and they always said nothing would happen but he ended up leaving me for her so I'm worried the same would happen again. I told him this and he just says I got nothing to worry about but that's what my ex used to always tell me. What do you guys think?