Did you see a movie called "There Will Be Blood? Can you explain something that I can not figure out about the story?

I rented this movie on Netflix. It is pretty good.

If you saw the movie, you will know what I am talking about.
A young man, Paul Sunday, approaches the oil guy, Daniel, and offers to sell him info about where he can find lots of oil.
Daniel buys the info and discovers a huge oil field.

Paul Sunday disappears.

In his oil adventure, Daniel meets Paul Sunday’s brother Eli, an ambitious preacher, who is played by the SAME actor.

Are they really twin brothers, or was Paul actually Eli, playing Daniel in order to get him close to his church so he can tap into the oil money?
Unless I missed something, I don’t think it is made very clear in my viewing of the movie.

I suppose it is not a big deal, but I can’t let it go.