The inciting incident of this story happens immediately, if not before the story. An inciting incident will introduce the problem that the novel or story tries to overcome or solve. In this case, the moment of the crash which leaves the boys stranded on a deserted island would be the inciting incident. This moment actually happens before the book occurs. If you need to have something within the text, I would argue that the moment the boys realize they are without adults should be the next best event to call the inciting incident.
A first turning point in the novel occurs when Jack breaks from the rest of the boys. In this moment, it seems as if it is possible that death is closer than a solution to their problem of being stranded. This occurs in chapter 7.
A next turning point occurs when Simon dies in chapter 9 trying to tell the boys that the Beast is not real, but is the evil that is a part of all of them. It seems as if they will never understand the evil that is beginning to rule them.
When Piggy dies in the next chapters, all hope seems lost. This is the last turning point that could be considered the climax because at that point, it seems like the boys will never recover from what savagery has done to them. This savagery relates to the inciting incident because the absence of parents to control behavior led to these terrible acts.
This is answered comprehensively (there's a video too) at http://www.clickok.co.uk/Inciting-Incident-versus-Call-To-Adventure-Resolved.html