Brother against sister.

BTW, didn't you throw a knife at one of your 3 sisters ;)

I only grew up with my brother. But honestly I believe twins fight more than any other siblings. Based on my own twin experience, and friends of mine that are twins. It's because you see that twin EVERYWHERE. You'll have classes with them, you're exactly the same age so you're liable to have mutual friends. It's frustrating.

Okay, I didn't get as good a response as I was hoping for, but at least the data is consistent, and supports my theory.

My theory is that brother-sister conflicts would be more serious and more common than either brother-brother conflicts or sister-sister conflicts. This is based both on personal experience, with my sisters growing up, and by observing my own kids, but also on evolutionary pressure.

In a lot of species that produce multiple offspring at the same time, and one or more offspring are smaller and weaker than the rest, the stronger offspring will pick on the weaker offspring, and will out-compete with the weaker offspring for food, to the point where the weaker offspring generally dies. A lot of the time the parents themselves will be the ones to abandon, or even sometimes kill, the weaker offspring.

This makes a certain amount of sense since, if food is scarce, it's in the best interest of the species concentrate resources on the stronger offspring.

You also don't see this with siblings that are about the same level of health, presumably because the chance of injury for both siblings out-weighs the potential benefit of having less competition, but also possibly because there's no real benefit to the species in removing other-wise healthy individuals.

We see echos of this in our own society as well. Historically, school children are well known for their cruelty to weaker classmates. Today's schools do a lot better job than when I was kid in suppressing bullying, but it still happens.

Since humans usually produce only one child at a time, and since subsequent children are born before the older ones move out, we perpetually have a situation where the older children are significantly stronger than the younger ones. From the older child's perspective, it might make sense to pick on the younger kids, and out-compete with them for resource, because it will improve your own chances for survival, but from the parent's perspective, and the perspective of the species, these younger siblings are perfectly healthy in their own right, and have just as much chance of evolutionary success if they live to adulthood as their older siblings, so you really want to ensure that they don't get picked on, and that they get their fair share of the resources.

And this is exactly what we see. Older siblings almost always pick on their younger siblings, and parents, and society in general, almost always try to get them to play fair, and maybe try to offset it a little bit by giving the younger siblings a little more attention when the older siblings have been to rough.

Now, to get to the whole brother-brother, sister-sister thing. One advantage for the older sibling of not being too rough with his little brother (and the same goes true for sister-sister relationships), is that that little same-sexed sibling can grow up to be an important ally in adulthood. However, the same may not be true for opposite-sex siblings because the sister, or brother, depending on your society, is likely to be married off to another family, or even tribe, and may even someday be a competitor. Because of this, the evolutionary pay-off of being nice to your opposite-sex sibling is going to be weaker to non-existent, and that's why I theorized that brother-sister conflicts would be more serious than brother-brother or sister-sister.

Some might argue that even same-sexed siblings will marry into separate families, and I can't provide a strong argument against that, but I remember from my Anthropology classes back in college that when individuals in tribal societies look for potential mates, they had strict rules concerning who could be a potential mate, and who couldn't, in order to avoid incest, and, depending on the society, either brothers would stick together, and sisters would go to different families, or the other way around, and even for the sex that went to other families, they could often go to the same family since the family that marries one sister could also marry the other.

Now the twin aspect of things is something I hadn't considered until I saw Saurbaby's post. In this case, they're both about the same strength, so that would tend to reduce the amount of conflict, but, as Saurbaby points out, her experience tends to support the opposite effect, due to more frequent interaction.

Anyway, that's my theory. I know it's a little long for most people, and I don't know how many people will really read it, but it was fun to write. :)

And for you religious people out there, being able to look at the world in a rational way, and figure out how it works, is one of the greatest pleasures in life (next to playing with your baby son as you're getting ready for bed). I think it's why most atheists are so against religious arguments. They take away your right to think for yourself, and force you to follow the doctrine of someone else. And what I can't figure out is why that guy, the guy who got to write the doctrine in the first place, was so lucky to land that job. In a world where doctrine is handed down to most people from someone else, it's the lucky few who get to the ones to hand down the doctrine.