Hmm, that's an interesting situation.
I came in as a tourist when I meant to live here and they did ask me how long I was staying, I said 6 months, then they asked how much I had in my bank, I said "Oh, Um... around $1000 or so" then they asked how I planned on supporting myself. I just said my parents would help.
They didn't ask for any documents. They didn't even ask to see my return ticket (thank god, because I didn't have one). I don't think they'll ask him for documents, but you never know.
I can't guarantee it'll all be fine because everyones experience is different, but I think you'll be okay. If they ask, all he needs to say is his sister will help if need be.
The thing to do is not give more information than they ask for. Only answer the questions they ask. He shouldn't have any worries about coming in as a tourist.
And have fun!