...those siblings at home is not absolute. In fact, when a strong figure presents an opinion, Simon tends to "clam up" and finds it difficult to present his thoughts and opinions to the group. This is exampled very well in his inability to tell the others what the real beast is.
All of this put together very much reminds me of the older brothers I see in school on a regular basis. They can be very protective of their younger siblings during the school day, especially those that have more than one in the building. However, once mom or dad is around them, they have a tendancy to give over that control and supervision to the higher authority, particularily if the parent has an overpowering personality. This provides a stark contrast to other characters on the island, like Jack, who I could easily picture as an only child with too much time on his hands at home.
I don't believe we're ever given Simon's exact family history, but I think there are certainly some family life things we can infer from the actions of his character. I think we can safely assume that Simon is an older brother to multiple siblings at home, he is given large amounts of responsibility in watching those siblings, and he is subject to some form of strong authority at home, either a parent or some other caretaker.
These may seem like contradictory statements, but let's look at how Simon acts in the novel. He is, by far, the most helpful boy on the island. Whenever someone needs help, he is the first to give it. Simon helps Ralph build the shelters, gives Piggy his glasses back when Jack takes them away, gives Piggy meat at the first feast when Jack denies it, grabs fruit for the littleuns when they can't reach it, and provides moral support when Ralph starts questioning his own motives.
Simon is also one of the more brave characters on the island which may indicate his natural instinct to protect those around him. He has no problems standing up to Jack to when he feels Jack is wronging someone (particularily Piggy), and makes multiple trips through the jungle by himself, an action that should frighten any twelve year old.
Aside from all of this, though, Simon shows a tendency to heed any direction from authority figures, in this case Ralph and Jack, something that suggests his protection and supervision of...