Marianne Walsh and her sister, Megan Putman, keep track of whose kids their mother babysits more. They also compete with each other over parenting styles -- Ms. Walsh is strict, Ms. Putman is laid back -- and their weight.

'My kids play more instruments, so I am winning in piano,' says Ms. Walsh, 38, the younger of the two by 13 months. 'But she won the skinny Olympics.'

Adult sibling rivalry. Experts say it remains one of the most harmful and least addressed issues in a family. We know it when we see it. Often, we deeply regret it. But we have no idea what to do about it.

Ms. Walsh and Ms. Putman have been competitive since childhood -- about clothes, about boyfriends, about grades. Ms. Walsh remembers how in grammar school her sister wrote an essay about their grandfather and won a writing award. She recited it at a school assembly with her grandpastanding nearby, beaming. Ms. Walsh, seething, vowed to win the award the next year and did.

Ms. Putman married first. Ms. Walsh, single at the time, clearly recalls the phone call when her sister told her she was pregnant. 'I was excited because this was the first grandchild. Then I got off the phone and cried for two hours,' says Ms. Walsh.

Ms. Putman, 39 and a stay-at-home-mom in Bolingbrook, Ill., remembers that she too felt jealous -- of her sister's frequent travel and promotions in her marketing career. 'The way my parents would go on and on about her really made me feel 'less than,'' Ms. Putman says.

Ms. Walsh eventually married, had a son and named him Jack. Seven weeks later, Ms. Putman gave birth to a son and named him Jack. The discussion? 'That was always my boy name.' 'I never heard you say that.'

Sibling rivalry is a normalaspect of childhood, experts say. Our siblings are our first rivals. They competed with us for the love and attention of the people we needed most, our parents, and it is understandable that we occasionally felt threatened.

Much of what is written about sibling rivalry focuses on its effects during childhood. But our sibling relationships are often the longest of our lives, lasting 80 years or more. Several research studies indicate that up to 45% of adults have a rivalrous or distant relationship with a sibling.

People questioned later in life often say their biggest regret is being estranged from a sister or brother.

The rivalry often persists into adulthood because in many families it goes unaddressed.

'Most people who have been through years of therapy have worked out a lot of guilt with their parents. But when it comes to their siblings, they can't articulate what is wrong,' says Jeanne Safer, a psychologist in Manhattan and author of 'Cain's Legacy: Liberating Siblings from a Lifetime of Rage, Shame, Secrecy and Regret.'

Dr. Safer believes sibling rivals speak in a kind of dialect (she calls it 'sib speak'). It sounds like this: 'You were always Mom's favorite.' 'Mom and Dad are always at your house but they never visit me.' 'You never call me.'

'It's not the loving language that good friends have,' Dr. Safer says. 'It's the language of grievance collection.'

It's hard to know what to say in response. 'You are afraid that what you say will be catastrophic or will reveal awful truths,' Dr. Safer says. 'It's a lifelong walk on eggshells.'

Sibling discord has been around since the Bible. Cain killed Abel. Leah stole Rachel's intended husband, Jacob. Joseph fought bitterly with his 10 older half brothers. Parents often have a hand in fostering it. They may choose favorites, love unevenly and compare one child with the other.

Dr. Safer draws a distinction between sibling rivalry and sibling strife. Rivalry encompasses a normal range of disagreements and competition between siblings.

Sibling strife, which is less common, is rivalry gone ballistic -- siblings who, because of personality clashes or hatred, can't enjoy each other's company.

Al Golden, 85, chokes up when he talks about his twin brother, Elliott, who died three years ago. The brothers shared a room growing up in Brooklyn, N.Y., graduated from the SUNY Maritime College in New York and married within a month of each other in 1947.

Yet Mr. Golden still remembers how their father often compared their grades, asking one or the other, 'How come you got a B and your brother got an A?' He rarely missed a chance to point out that Elliott wasn't as good as Al in swimming.

When the boys were ready to get married, he suggested a double wedding. Mr. Golden put his foot down. 'I shared every birthday and my bar mitzvah with my brother,' he said. 'I'll be damned if I am going to share my wedding with him.'

Elliott Golden became a lawyer and eventually a state Supreme Court judge. Al Golden went into the mirror business, then sold life insurance. He says he always envied his brother's status and secretly took pleasure in knowing he was a better fisherman and owned a big boat. Once, Elliott asked him, 'I am a lawyer. How come you make more money than me?' Mr. Golden says. 'He meant: 'How come you are making more than me when you are not as successful?' But it made me feel good.'

One day, Mr. Golden says, Elliott accused him of not doing enough to take care of their ailing mother. After the conversation, Mr. Golden didn't speak to his brother for more than a year. 'It might have been the build-up of jealousies over the years,' he says.

His brother repeatedly reached out to him, as did his nieces and nephews, but Mr. Golden ignored them.

Then one day Mr. Golden received an email from his brother telling a story about two men who had a stream dividing their properties. One man hired a carpenter to build a fence along the stream, but the carpenter built a bridge by mistake. Mr. Golden thought about the email then wrote back, 'I'd like to walk over the bridge.'

'I missed him,' Mr. Golden says now. 'I never had the chance to miss him before.'

Dr. Safer says brothers' rivalries often are overt, typically focusing on things like Dad's love, athletic prowess, career success, money. Women are less comfortable with competition, she says, so sister rivalries tend to be passive-aggressive and less direct. Whom did Mom love best, who is a better mother now.

Brothers often repair their rivalries with actions. When women reconcile, it's often through talking. Ms. Putman and Ms. Walsh have learned to stop arguments using a trick from childhood. When a discussion gets heated, one sister will call out 'star,' a code word they devised as kids to mean the conversation is over. The sister who ends it gets the last word. 'You may still be mad, but you adhere to the rules of childhood,' Ms. Walsh says.

For some years, the two didn't socialize much. But when Ms. Putman's husband died last fall, Ms. Walsh, now a stay-at-home-mom in Chicago, helped plan the wake and write the obituary. Arriving at her sister's house one day before the funeral, Ms. Walsh found her in bed, crying, and climbed in next to her. The sisters said, 'I love you,' and Ms. Putman says she realized she was going to be OK.

'Lying there, I felt that if I've got my sister, I've got my strength,' Ms. Putman says. 'She is my backbone.'

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Fix the problem by addressing it head-on, says psychologist Jeanne Safer.

•The first step is to think. Who is this person outside his or her relationship with you? What do you like about your sibling? Remember the positive memories. Identify why you think the relationship is worth fixing