Really the half sister makes the most sense. He would have access to her birth certificate more easily than to any other person’s.
I think HIS certificate says he is Barry Dunham and that his parents were not married. No big deal today, but he has created this illusion and Barry Dunham doesn’t fit. His problem now is that he spent years creating the image and hiding the truth so if the truth comes out now it might hurt him with the ones who are enthralled by the image.
That is what I thought as well. He was born Dunham. Then he was adopted and had his name changed to Barry Soetoro. Then sometime latter on he changed it to Barrack Obama.