Is the middle child forgotten? Is the eldest always bossy? Róisín Inglelooks at a new take on turbulent sibling relationships.

Being from a family of eight children, the subject of psychologist Dorothy Rowe's latest book, her 12th, made me squirm.

Anyone who has a brother or a sister or a handful of both will be keenly aware of the murky depths those "blood is thicker than water" relationships can plumb. From that perspective, My Dearest Enemy, My Dangerous Friend: Making and Breaking Sibling Bondswas never going to be a barrel of laughs.

Flicking through the book, the familial flashbacks come thick and fast. There was the time I convinced my youngest sister that even though she was physically attached to it, I had legal ownership of her right arm. Or those months when I persuaded my younger brother that I was a witch and that he was helpless while under my spell. A dangerous friend, indeed.

I've felt guilty about these and other examples of atrocious sibling behaviour for years, but talking to Rowe I began to feel less freaky and more, well, normal.

"From speaking to so many siblings while researching the book it's clear there is a lot of guilt around how they treated each other as children, but feeling hatred or resentment towards a sibling is not peculiar," she says in her soothing Anglo-Australian burr down the phone from her home in London.

Having already brought what author Sue Townsend described as a "calm voice of reason in an increasingly mad world" to subjects including depression and friendship, 76-year-old Rowe says she wanted to write about siblings because there is a "big blank" in the work of psychologists in this area. "I am hoping the book will encourage people to look more closely at sibling relationships, as closely as they do at the hugely important parent-child relationships.

"Psychologists have tended to stay away from the study of siblings because, unlike with the parent-child relationship, there are so many things to take into account: ages, genders, positions in the family, whether they are introverted or extroverted, so it's impossible to draw a neat pattern," she says.

But it's a mistake not to explore these relationships just because they are less tangible. "Siblings know each other incredibly well," says Rowe. "You know even when you are quite tiny what it is that your sibling views as tremendously important in the world, and from an early age you can support that or you can undermine it . . . These are lifelong relationships that have a huge bearing on who we are."

Having endured a troubled relationship with her sister while growing up in her native Australia, Rowe always knew she would tackle the subject, but says she hasn't been brave enough until now. Rowe's sister Myra resented her from birth and never missed a chance to ridicule or, as Rowe puts it, reduce her "to dust beneath her feet".

"I was always too scared to write about her," she says. "We had a difficult relationship; I was born on her sixth birthday and she never forgave me for that. When I was born she was sent away for a few weeks to an aunt's house and she blamed me for the separation from our mother." Her sister bossed her around, undermining her at every turn, while her mother beat her and, as a result, young Dorothy grew wary of expressing emotion in case her vulnerability was exposed.

"Like so many people, I didn't talk about my sibling for years. We don't tend to do that if the relationship is bad because the sibling can do us damage," she explains.

In the book, she explores issues that are often overlooked when parents regard the relationships between their children: the different ways introvert and extrovert children experience the world and each other; the pressure of "being good"; the divergence of childhood memory; and, most movingly, the emotional carnage left behind when one sibling dies.

Rowe responds to all the obvious questions - "is the middle child the forgotten one?", "is the eldest always bossy?" - with a disclaimer that sibling relationships are as varied as snowflakes. She seems allergic to generalisations and, while this might make her answers less soundbite-worthy, it lends an authenticity to her musings.

There are, she will concede, some patterns which tend to recur. "If you are the first-born you form a picture of yourself and the world; one day that picture is destroyed by this strange little thing who turns up and suddenly your mother is paying more attention to it than to you. It feels as though we are falling apart. I call it annihilation and it takes a while to get the picture of ourselves back together again."

Rowe says this is what happened to her own sister when she was born. I can't help wondering whether she fears Myra's wrath - she is 82 now - after she reads the scathing account in the book of their relationship.

"She may read it, although I haven't given her a copy of the book. The thing about my sister is that she doesn't remember anything that goes against the picture of how she wants to see herself and her life, and everybody around her finds this difficult," she says. "When my sister and I have a conversation about something that happened years ago, she will say things like 'oh you've got such a wonderful memory, you remember your childhood'. She has blocked out whole portions of her life that don't suit her. It's a method of defence that Freud called repression."

Understanding why a sibling behaves the way they do can be useful but this understanding does not protect you from being hurt. "Just because you know why someone behaves badly, and as a result you have compassion for them, does not mean they can't hurt you," she says. "You can feel really sorry for a lion in the zoo but it's not a good idea to get over the fence and give him a cuddle. The results can be savage."

Rowe now understands her sister far more but that doesn't lessen her capacity to wound. She writes in the book about taking her sister and her husband to lunch in London's Fortnum & Mason when they were both in their fifties. Her sister was talking about toilet training a kitten and it sparked a memory of when Dorothy was a baby. Her sister said: "You were propped up in your high chair and you wet and it went all over the seat and dripped on to the floor. Mother rubbed your nose in it." This story explained a lot about the way Myra learned at the age of six, from their own mother, how to regard her little sister.

AFTER ALL THESEyears does she forgive her sister? "Forgiveness is a very complex issue," she says. "I was very badly treated by my ex-husband, he went off with someone else, but his behaviour freed me and allowed me to pursue a life that's been very interesting and rewarding. When I think about him now, which isn't very often, I don't feel angry. Forgiveness is irrelevant.

"When I go out to Australia every winter to see my son this involves meeting my sister and I get dragged back into the family stuff and I experience that same inner turmoil. So I would be lying if I said yes, I forgive her, but I don't feel angry and bitter about her any more. The best you can hope for is to not be consumed by bitterness and anger and resentment, to be free from all that."

I express mock disappointment that she is not marketing the book as some kind of sibling cure-all and Rowe laughs her slightly wheezy laugh, a throwback from a childhood lung disease for which neither her mother nor her sister tried to help her get treatment.

"You can use the book as consolation, as a way to understand," she says. "But it is not a cure. Unfortunately, there is no cure for families."

• My Dearest Enemy, My Dangerous Friend: Making and Breaking Sibling Bonds by Dorothy Rowe is published by Routledge, €14.95.