Is my boyfriend and his sister too close

I've been dating my boyfriend a couple months. He doesn't have a relationship with either of his parents and was pretty much raised by his sister since he was 5. They have a 9 year age difference. He will talk or text her all the time and goes over to her house at least once a week. He's 17 almost 18 and lives with a friend because he wanted to. I'm 22. I have met his sister and I have no problem with her. She's very motherly. But it seems when we go over there he always has to hug her when we get there and when we leave. And he gives her big hugs, not side hugs. He tells his sister I love you, but tells me Love you. He actually is thinking about moving back in with his sister for Junior year which is weird to me since he will be 18. I had also went through his phone and saw text messages between him and his sister. *Trigger warning* he was talk about rape. From what I read it seemed like he was raped when he was 4. He never once told me that. I feel like if you're having trauma this is something you go to your GIRLFRIEND about. He's never told me about that and I'm his gf but he will talk to his sister about it? Am I just crazy?