LESLEY: You’ve written a powerful, emotional book, Apples and Oranges, about what it’s like when you don’t get along with your sibling. There’s guilt, there’s longing … and almost uncontrolled anger. First off, tell us why you and your older brother Carl, your only sibling, were at odds.

MARIE: That’s a mystery. And it’s the mystery of so many brothers and sisters. A subject that for some reason is rarely talked about. How do two children grow up so close in a family and then become foreigners in adult life?

There were the obvious differences from childhood. I was the younger sister playing my Joan Baez records to annoy him and he was probably the only one in our elementary school who was a Republican at 10. He was also an early member of the National Rifle Association!

So, we fought all the time. Our mother called us apples and oranges. We were different.

LESLEY: As I recall from the book, he was “red state” to your “blue state,” you the NYC sophisticate. You disagreed on religion too. All the hot spots. Did you agree on anything?

MARIE: At first, I thought absolutely nothing. I was wrong. I thought we were polar opposites. I was wrong about that too. He was a control freak, an obsessive, who would take a look at my messy desk and say, “How can you ever get anything done?” He posted signs up: A Failure to Plan is a Plan to Fail. When he was young he used to polish his shoes and line up his shirts just so. And secretive? Let’s not go there.

I was the noisy younger sister. When we would get into battles over politics, he would lose it and say, “You and your friends, the New York libs.” I mean, really.

LESLEY: I always thought sibling relationships were far more formative and important in how we develop than anyone talks about. The book is both a riveting story, and an examination of sibling rivalries that never stop. You went to conferences, interviewed experts. What did you learn about the brother/sister or sister/sister wars?

MARIE: I learned a lot. A recent study suggests that our relationships with our brothers and sisters is the dark matter that defines us. For years, psychiatrists and family therapists more or less ignored this. Imagine that.

By age ll, you’ve spent more time with your siblings than your parents or friends. Forty percent of us have relationships with our siblings that are distant and/or infuriating. Many of us are like moose with our antlers locked together.

There is no question that the closer you are with your siblings the more you feel content in your life. There are studies about that too. So, for me, the question that became my obsession was: How to make this better? How to reframe?

LESLEY: Before I ask you specifically about you and Carl, I’ve heard you talk about Hillary Clinton and Barack Obama as siblings. Explain that.

MARIE: This fascinates me. Think of this as the iceberg of all sibling relationships. That is a term therapists use. Icebergs. Many of us have them. I wonder, watching Hillary and Obama as they battle each other, whether they are trying to cope with their own sibling issues. Are there icebergs here?

Hillary is a classic firstborn, bossy and determined. She had two bratty younger brothers she both protected and stomped on. She still does with Tony and Hugh. Those smarmy presidential pardons, what was that about except sibling stuff? Is that connected to Hillary and the piece of her with Bill Clinton that reminds many of Bonnie and Clyde? Look at that gleam in her eye when she debates Obama. It is like she’s in a big-sister devil cult, swatting a swarming fly.

And then there’s Obama. He was raised more or less like a golden-child-only prince. Yes, he has siblings, halves and steps, but they are much younger — or older. And a continent away. He had siblings on demand. This gives him lots of confidence, but he is not so good at the parry and thrust and wit you get in the sibling romper room. In this way, he is no JFK.