They are our closest friends, our most intense rivals. They delight us … and infuriate us.
"They know your buttons to push, and how to get at you when they really want to," said Jim Bratton of Atlanta.
Like it or not, we may know our siblings better than anyone else in the world. Studies show that by age 11 we spend around a third of our out-of-school time with our brothers and sisters.
So no wonder they are a major force in determining who we are and how we behave:
"The sibling relationship is the longest-lasting relationship that many of us have," said Brenda Volling. "It's in place long before we meet a spouse or a partner, and it continues long after our parents have passed away."
But Volling, a developmental psychologist at the University of Michigan, says sibling interaction has been studied far less than parent-child or spousal relations.
So she is trying to understand what happens at very early stages in a family.
She observes behavior at sessions like one we visited, which she set up to show how it's done.
She asks a parent to focus on one child only . . .
"Now it's her turn, then it will be your turn."
Then the second child gets most of the attention.
And Volling also watches to see how the children interact when left to themselves.
"The relationship with siblings is very emotional," Volling said. "You can see it go back and forth from kids who are hugging and sharing and loving each other up, then two minutes later ready to, you know, topple over each other, wrestle each other."
The object of all of this is to try to understand how an older child reacts when a second child is born.
"Not all children react exactly the same," Volling said. "And that's one of the things we're trying to uncover. But by the time the baby's been in the household for four months, kids seem to have adjusted."
Researchers say that even sibs who argue a lot as children can become close friends as adults.
"When we were younger, we used to have fall-out, drag-out . . . " said Jill Jackson. A sister continues, laughing: "Put a hole in the wall!"
That's right! Once upon a time the Jackson sisters of Windsor, Conn., were a brawling bunch . . .
"My father came home and said, 'What is this? You're girls! This isn't supposed to happen!' said Tammy.
"I feel as though our relationships are priceless," said Tammy.
Tammy and Jill even live next door to each other.
And the Jacksons admit that they do fall into those birth order stereotypes we've all heard so much about . . .
"I feel responsible for them," said Jill, the oldest.
"When something goes wrong it's kind of, I'm the one that it gets filtered through," said middle child Wendy.
"I am probably the one that ventures out the most and does something different," said Dana, the youngest.
So does birth order really have an impact?
"The family's like a chessboard, and birth order's sort of like the knight on the board," said Frank Sulloway. "It's more important than pawns, but less important maybe than the king and the queen."
Frank Sulloway, of the University of California, Berkeley, is a winner of the MacArthur "genius" grant, and considered the country's leading authority on birth order.
He says that because older siblings tend to take on more responsibility in a family, they also excel in school:
"They do their homework, they get good grades. And hence they get into Harvard, Yale and Princeton," he said.
"What about the middle child? I mean, I have to admit, I've got some vested interested in this, I am a middle child," said Braver.
"Many studies have shown that middle children are more peer-oriented," said Sulloway. "They're less attached to the family and they invest more in their peers.
"Most people know that younger siblings are the sort of sociable and more reckless sibling, the daredevil and maybe the jokester of the family. And when we actually look at the empirical record, that's what we find."
Gender is important in the family dynamic, too. Relationships differ depending on whether it's brother to brother, sister to sister, or sister to brother. And of course genetics is a key factor in making us who we are. We share some (but not all) of our genes with our siblings.
Which helps explain why some of us have similar talents, and others succeed in completely different fields.
And as for sibling rivalry? Well, it famously began with Cain and Abel, and remains the stuff of popular culture. Remember "Everybody Loves Raymond"?
But a poll we conducted found only 22% of Americans admit to feeling competitive with siblings. Three quarters (74%) say mom and dad treated all the kids equally. Almost half of us (49%) say we're extremely close to our brothers and sisters.
And though more than a quarter of us (28%) say we've stopped talking to a sib for a while, only 12% say it's gone on for more than year.
For psychologists like Peter Goldenthal, uncovering the roots of sibling stress can be tricky:
"Do people actually come to you talk about relations with their siblings?" asked Braver.
"Sometimes," said Goldenthal. "More often they come for something else, and that emerges sometimes quickly, sometimes not so quickly."
What drives sibling quarrels between grown-ups?
"It very often involves a very serious illness or the death of a parent, where somebody is so hurt and so angry, that they cannot get beyond it," he said. "And when it doesn't involve that, the second one is money."
But even if things don't get quite to the level of the Corleones in "The Godfather Part II" ("Fredo, you are nothing to me now. Not a brother, not a friend. I don't want to know you or what you do."), Goldenthal says that healing a breach with a sib can be difficult.
"It's easier to lose a pound than to improve a relationship with a sibling, but you get more out of it," Goldenthal said.
Is there any evidence to show that people are happier in their overall lives if they have good relationships with their siblings?
"Of course anybody who has that type of support in their life is going to, I would venture to guess, feel happier and more satisfied with their life," said Volling.
Tammy Jackson concurs: "It's the greatest blessing in life to have sisters. My life has always been full."