A lot of people have this misconception about evolution. Evolution is not a theory, its a fact. What Darwin came up with was a theory to explain the phenomnon of evolution. So Darwin’s theory was called the theory evolution by natural selection. There were other theories to explain evolution but Darwin’s was the one that made the most sense.
Primitive humans might not have had any reservations about the sister/brother mating just like other close relatives of humans in the animal kingdom. Notice that i didn’t use the word marriage.
I’m not sure about sister marrying brother in Hazrat Adam’s shariat. I would however, like to know where this concept is coming from along with them being born twins. As far as i know, there is nothing in the Qura’an in this regard.