I agree with all of you. And C'est Moi, you're correct, I'm the tag along, not his sister.
There will be a conversation to follow but I started with an email to him today. I could have been more blunt than I was but I chose to be as non-critical as possible. Why? Because nothing will change between them regardless of what I say. A relationship with me or any other woman will never rise to the level of importance as the one he has with his sister.
He's a huge collegiate football fan and several of his favorite teams are playing today so I doubt he'll even check his email until later tonight or sometime tomorrow. I'm sure we'll talk over the next day or two but, in the meantime, he has an email waiting for him. I told him that there isn't room for me in this relationship and there's no path forward. He's content to spend his time with his sister in the little world they've created and I am a disruption. I said more but this was the gist. If I was a betting person, I'd wager a lot that his sister will be the first person he talks to after reading the email and that he may even forward it to her.
I spoke to a good friend yesterday about him and she summed it up perfectly. She said "unfortunately he's damaged".