If you remember, Walter had told Jesse that Tyrese had stolen the cig with the ricin from him since he had such easy access to their stuff in the lab, and then gave it to Gus to use on the child. He also manipulated Jesse into the idea more by citing examples of Gus killing children.
The whole time, Yuell, in reality stole the cig from Jesse on behalf of Saul for Walt. Then Walt planted the cig in the robot vacuum cleaner and Jesse finally was able to rest his mind that Walt had nothing to do with it and he did almost kill him for nothing. Even though we all know thats not true...just how damn cold and smart Walt is.
Pretty sure that timeline above explains everything and why the Ricin cig ITSELF had nothing to do with Brock and its all Jesse knowing now that Tyrese never took it and that Gus was innoncent the whole time, and that Walter had now for sure been proven to have SOMETHING to do with what happened in the Brock situation atleast
