Boyfriend’s "best friend" seems like more than just a friend

Hello,
So I’m confused on whether my boyfriend’s “best friend”, who is like a “sister” to him, is really what he says she is. Because his story doesn’t really add up, and this is what it is:

He has this “best friend” who he said he’d known from childhood, and they talk on the phone at least once a week. I never gave it a second thought until recently when he told me that he never even MET her until 1 year ago. He is 23 now…that doesn’t seem like a childhood friend to me if he hadn’t MET her until he was 21. He said he knew her first from a friend when he was 14 and was supposed to meet her, didn’t get to…Then when he was 18 he started online chatting with her frequently and became fast friends. I called him out on his intentions of sleeping with her– he admitted that initially he did when he was 18, but quickly changed his mind b/c she slept around so much…(but knowing my bf’s past, sleeping with loose girls never dissuaded him from doing it, he just doesn’t like having a relationship with them, double standard, right? can sleep with them but can’t marry them)

Up until now (the past 15 months) he has passed of his “best friend” as a “sister” and has emphasized that he “never thought of her in sexual way”. I know that after time your feelings on someone can change, and maybe he did lose interest in her in that way, but he could have just been open about it since I confronted him several times about his past with her. And then I can’t ignore the fact that after he broke up with his ex of 8 months, he went to visit said “best friend” for the FIRST TIME, even though she lived 6 hours away.

I don’t understand how you can be best friends with someone you had never met, AND drive 6 hours to meet them for the time and say you went with no intentions of sex, when that’s what pushed you to start talking to them in the first place. (especially for a guy, they don’t invest so much time and effort in a girl unless they see a return factor).

He says he was really depressed from the break up and just needed a change/vaca, and denies doing anything with her. But why see this girl? I’m convinced they did something physical.

Anyway, maybe I’m over-reacting with this new information. It just feels like he’s masking something. And it ticks me off too because despite me disliking her, I never once made it an issue that he was friends with her. Once in a while he gets suspicious that I’m cheating on him and I’m pretty sure she influences him with that…She’s married and has cheated on her husband with other guys (which is why I dislike her so much), and then she says little things to my bf like “oh your gf still hasn’t been with any other guy? weird”–these statements make him suspicious of me (and its irritating to deal with).

I don’t see WHY he keeps talking to this girl, she doesn’t really have much to offer him from what he tells me, (she mostly complains about her marriage and talks about her cheating escapades). He says they talk at least once a week (she lives a few hours away) Maybe he uses her as an outlet for weird fantasies or something, I Don’t Know.

But How should I even react to this? Is it reasonable for me to believe they have had a sexual past? I’m really confused!

Thanks in advance for any replies!

-Poly