I think I missed something... (SPOILERS)
When did Dwight learn for certain that William was his half brother? I pay very close attention to movies, but I don't remember when this particular detail was revealed.
shareTeddy Cleland told him after Dwight opened the trunk. Teddy had no reason to lie seeing as how he was in possession of the Ruger at the time of the revelation.
shareNO, Dwight asks "Who was the kid in the limo ?" and Teddy answers "It's a kid. Just a kid"
At the end when he lets William go free Dwight says "He's the boy my father had with your mother". But it's NEVER said by anyone from the Cleland family. Just what Dwight concludes by himself.