‘90s FAMILY : Ties That Bind : We play with them, fight with them, seek advice from them. No wonder siblings play major roles in the people we become.

After one of Jane Mersky Leder’s brothers committed suicide when she was in her early 30s, she began to wonder about the underexplored sibling connection in her own family, and in families in general.

“Robin was my best friend, my ally. He was the sibling who idolized me,” Leder said of the brother to whom she was closest in both age and relationship. “Years after his death, as I turned 40, I wanted to have a closer relationship with my sister and bridge the gap with my other brother. I began to explore my personal feelings about sibling relationships.”

What Leder discovered served as material for a book, “Brothers & Sisters: How They Shape Our Lives” (St. Martin’s Press, 1991). It examines how siblings serve as a powerful molding force in a number of ways, such as selection of careers, mates, friends and development in interpersonal skills.

Throughout history, beginning with the biblical story of Cain killing his brother, Abel, the single most emphasized element of sibling associations has been sibling rivalry, leaving other intimacies and influences ignored, said Leder, who lives in Evanston, Ill.

“There’s been a reigning theory that parents are our major shapers and that siblings are minor characters,” Leder said. “It’s only been recently that professionals have been saying this is an important relationship. These are the people you shared the same space with every day and every night for all those years, often spending more time than with parents.”

Some experts have described the sibling relationship as one like a first marriage, where issues of intimacy are initially learned. Siblings are our first peers. And looking back at sibling connections--whether good or bad--could be quite revealing to adults.

“We may be leaving the nest as adults to do our own thing,” said Kevin Leman, a Tucson psychologist and author of “The Birth Order Book” (Dell, 1985), “but a lot of us are taking our brothers and sisters with us.”

Leman was the youngest of three siblings; he found that his inability to keep up with his older sister and brother made him more determined, harder-working and, inevitably, a success. In his work, Leman emphasizes that a person’s order among siblings plays a large role in determining personality.

“You can see adult influence versus children’s influence when you look at the youngest girl who has three older brothers. Now, here’s a woman who will understand men,” Leman said. “The oldest male with younger sisters is going to be very protective of women.”

Leder said her first career choice was teaching because of the joy she experienced helping her brother Robin. Her first boyfriends were selected based on their similarities to him. Her inability to handle confrontations stemmed from the standoffish nature of her other brother, John. Her warm connection to her younger sister led her to positive relationships with women friends.

The sibling influence was captured on film as early as toddlerhood for Lauri Carr-Brodie, 42, who said her identical twin--who was born first--”played dramatically on my personality.”

“If you watch our old home movies you can see it. She would instruct me how to do something and I would do it,” said Carr-Brodie, a Santa Clarita marriage, family and child counselor. “It took me 20 years to step out of the role I was cast in. I played the follower, while she had the leadership role.”

Although Denise Avaldivia, 23, of Newhall, admitted that she and her brother and roommate, Tim, 26, would probably need time and distance to assess how they’ve affected each other’s lives, some influences are already apparent.

“He’s always intentionally told me I couldn’t do things, just to egg me on, and I’d become even more determined to do them. For example, he said I would never get my cosmetology license, something I’d always wanted to do as a kid. I got it before I graduated from high school,” she said, adding that her influence as a younger female sibling can be seen in her brother’s behavior.

“He does things to protect me. Whenever he knows I’m going to be home alone, he locks every window, gives me a flashlight, baseball bat and his cordless phone.”

Having her brother as a roommate has other advantages as well, she said. “I can tell him things that I couldn’t tell anyone else.”

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Age spacing, gender of siblings, access to siblings, family patterns, ethnic patterns and genetics are all ingredients in the sibling stew, Leder said.

“It’s so complicated. That’s why many researchers have given up when they start looking at this area,” she added.

The seeds of sibling relationships are planted very early, and how they grow makes a big impression on each child.

“A sibling is a person the most like you and the closest to you,” said Irene Goldenberg, a family psychologist with the UCLA Neuropsychiatric Institute. “If the childhood relationship is good, then it can be one of the most supportive relationships into adulthood. If it is bad, it can be scarring and devastating into adulthood.”

Bad blood between siblings may occur for a number of reasons, but parents do play a role, Goldenberg said.

“Sibling rivalry is a negative reflection of parents being inattentive, of playing favorites,” she said. “This plays into a sibling’s sense of how well she or he is liked.”

Parents should be sensitive to what’s happening between siblings, experts say. They may already be influencing certain behaviors that can have an impact in the sibling relationship.

Carr-Brodie, the mother of three children--ages 14, 10 and 7--said it’s important for parents to spend time alone with each child.

“It’s also a good idea to have a family session where once a week you sit down and talk about feelings and what’s going on in the family.”

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Siblings may grow up to be the best of friends or distant strangers. Sometimes the wounds of childhood never heal. But experts are quick to cite the benefits of a close adult sibling relationship.

Barring severe circumstances--such as incest or physical abuse between siblings--writing off the “longest human relationship” would be like “missing a body part,” Leder said. “It behooves us to work as hard as we can to iron out the wrinkles.”

Smoothing sibling relationships may begin with being completely open about buried feelings or resentments, but allowing one another the right to his or her own memories, which are often completely different of the same event within a family.

Many adult siblings reach a turning point in their relationship where they want to become closer. This usually happens in the middle of life, Leder said, when all the self-absorption of teen years and young adulthood has passed and people assess where they and their families stand.

“It’s a maturation process. At the bottom of all this is a sensing of our own mortality,” Leder said. “As we age, we seem to forget what bugged us and put more emphasis on the good aspects of the relationships.”

The value of those relationships runs deep with so much common ground, including memories and history, Leder said.

“If we were lucky and worked hard enough, these people serve as our connection to the past and path to the future. I can talk to my sister about things that I can talk to no one else about because who else cares?” she said.

Carr-Brodie said her competitive relationship with her twin sister has evolved into a warm friendship.

“She’s great to talk to,” she said. “We’ve had our fights, but we’ve always worked them out. She is a part of me.”

Certain life-marker events--such as the marriage of a sibling, the birth of a sibling’s child and, in particular, illness or death of parents--urge adult siblings to come together again. The result can be positive or negative, depending on the siblings.

“The events will dredge up a lot of old emotions. We can seize it as an opportunity to solidify the relationship or re-establish what bums we think these people are,” Leder said.

And while having a sibling as a friend may not seem a necessity, there may be comfort in the unconditional constant presence of a brother and sister.

“You divorce mates, neighbors move away, friends come and go. Your siblings are always there,” Leder said. “The words brother and sister stand for the height of intimacy in a relationship and can represent the best in all of us.”