Boyfriends best friend 😡

Recently my boyfriends best friend has been really sexual and kinky towards me (both over text and in person) and I have no clue what to do. He is my other friends brother so I went over to her house the other day and she was out so he invited me in to wait for her even though I said I would just wait in my car, but of course he insisted that I’d come in.

I was there for about 20 minutes (it was very awkward) so I decided to go to the bathroom and just like wait there for a bit. I was in the bathroom and he WALKED IN and said “oh I didn’t know you were in there” when he obviously knew I was because I said that I was going to the bathroom. Luckily I wasn’t doing anything except just stand there so it was fine but idk what to do, since this isn’t the first time that something like this has happened.

There are times that he comes up behind me and puts his arms around me and whispers in my ear, and my friend sees this but just thinks that he is being “friendly”. I even tell him that I’m a loyal gf and that he’s making me uncomfortable but he really doesn’t care..

I just don’t what my bf to think that I’ve been doing something wrong, and there’s always this fear I have that he would believe his best friend over me. We’ve been together for over 18 months and I really don’t want it to end over something like this. We don’t typically fight a lot but this is something that I feel like is worrying. What if he blames me and thinks that I was flirting, or that I’ve been seducing him? Do I tell him whats been happening?

I’m 17 and my bf is 19 so he should understand and trust me right