AITA Boyfriend’s “Sister-Like” Best Friend Is Making Me Uncomfortable. Am I Overreacting?

My boyfriend and I have been together for about 2 years. He has this female best friend he’s known since high school who he always describes as “one of the boys”. He admits he feels attached to her in a way he doesn’t with anyone else because she helped him during one of the lowest points in his life from years ago. but like he says she’s nothing but more than a “good friend and sister”to him.

About a month ago when we were chilling at home and going through his camera roll together. I accidentally saw a series of photos of this female friend kissing him on the cheek. He immediately got defensive. Later he explained he even forgot the photo exists and it was just a joke from the Christmas party at her house last year when we were on a break (for unrelated reasons). Before that they hadn’t seen each other for a year. I let it go. But two weeks later, I found out they had hung out again very recently. and he had more photos of her alone from that night. He insisted another guy friend was there even there’s no proof. Again I let it go.

What bothers me is that he has no other female friends—absolutely none. Just her. His social circle is small in general and all his other friends are guys. Even before we got together people from their friend group assumed they were dating because of how close they were. On top of that, I recently found out that they’ve started talking constantly Every day ever since he went to her Christmas party. They send each other photos and videos like 30 times a day. What raised a red flag for me was when he opened one of her videos in front of me. My gut instinct told me something was off. The way she acted flirty and playful was just not how a “one of the boys” girl would do.

When I asked him to set boundaries, he refused and said it wasn’t necessary because they’ve been friends for years and it would be weird to bring it up now. He’s relying on “unspoken rules” and trusts her enough to think she won’t cross any boundaries, even though those boundaries have never actually been set.

I don’t want to be controlling, but this whole situation makes me frustrated. We’ve been arguing about this for weeks, and he just does not see the issue or understand why I’m frustrated. Am I overreacting, or is this a valid concern? What should I do?