Brothers fight both for and against each other. Sisters are the same. A common phenomenon a parent of more than one child experiences is the infuriating inability of children to share things with their siblings. Furious shouts of ďItís MINE!Ē bounce off the walls of many a suburban house. Small angry humans stomp and sulk, while parents sigh in exasperation. A mother hands one son something, imagining stupidly that all her offspring might skip off happily and play with it, and that child then insists that the thing belongs exclusively to him, and he would prefer to break it than to share it with his siblings. Another son tries to play with the thing, and a fight is the immediate result.
One could put this down to the general nastiness of children, but this doesnít square with other observations. The same boy, when his friend from school turns up, is happy to let his friend play with the object. The rivalry with the brother is intense and violent, but not with the friend. It seems that the sibling rivalry is more important than the object fought over.
Stranger still, in the school playground the next day, one son has his packed lunch stolen by another boy. Discovering this, the sonís brother quickly finds the culprit, and in moments the lunch box is returned and the thief runs crying to one of the dinner ladies. Again, a thing (the lunch) is disputed over, but now the brothers are instinctively supporting each other.
The above may not always happen, but the tendency towards this behaviour is very strong. The general childish nastiness theory will not predict and explain all of it.
My guess is that the answer, as always, lies in the genes. The boys are following evolved instincts which tell them to compete for parental resources, and to co-operate against non-family foes. Both boys want their parentsí resources for their own ends. Parental resources are finite, and if one brother gets a large proportion of parental time, attention, and money, then this necessarily means that the other brother will be getting less. Children who competed for parental resources successfully did better than those that got walked over by their siblings, and went without. This explains the near fanatical way in which children watch out for fairness in resource distribution. When the cake is cut up at the party, each will resent anotherís getting a piece a tiny bit bigger than his own.
However, the genes of children in the past who did nothing to stop their siblings being bullied by non-related children, did not get copied so many times as those of children who stepped in and saw that their brothers were fairly treated. Brothers share genes, and so a gene will pass on more copies of itself if it can alter the behaviour of a person to act in the defence of other copies of itself. Copies of itself are likely to exist in siblings. Therefore, brothers will try to see to it that their siblings fare well against the trials of the big wide world, and by this method they will help their own genes. At home, parental attention is a grand resource which is shared amongst the siblings, and no one else, and so the shares are competed for by the siblings.
When the friend of the boy comes to visit, the boy is happy to let his friend play with his toys, because this is a method of securing non-family alliances, and such things are very useful for the boy as an individual. His brothers will not mind this, since they stand to lose nothing. But if one brother tried to win friends and influence people using the parentally-granted property of his brother, all hell would break loose.
Thatís human nature for you.