‘90s FAMILY : For Better or for Worse . . . : You used to fight. Now you may be best friends. Siblings have the richest relationship in the family.

As kids, our brothers and sisters knew just how to push our buttons. They were our first peers, first friends and first enemies. Sibling relationships often outlast marriages and frequently survive parents. It is our siblings who usually make up the longest running thread in a lifelong patchwork of people we know.

Many experts say the sibling bond is more telling than the parental one. Especially those adult siblings looking to settle old scores.

“If the issues when you were young were who has more crayons, the issues when you are adults are who has more cars,” says Jane Greer, a Manhattan psychotherapist, marriage and family counselor, and author of the book “Adult Sibling Rivalry: Understanding the Legacy of Childhood” (Crown, 1992). “The items may change but the issues are the same.”

Greer and other experts believe the seeds of rivalry come from many places, but are rooted in parents. “Rivalries are inherent because children will always compete for the attention and favors of their parents,” she says.

Justine Kaplan of Laguna Beach says she feels she’s been fighting 17 years to gain an ounce of the attention her father gives to her younger stepsister.

“I love my stepsister, but in terms of sibling rivalry there are times I really hate her,” she says. Her stepsister, now 22, is the child of her father’s remarriage.

“Even though I’m 34 years old, I’d like for him to treat me the way he treats her,” Kaplan says. “He gets on a plane to visit her in Boston, when he’s never visited me in any city I’ve lived in. On her birthday he lavishes her with gifts. On my birthday I get a check.

“She is really not to blame for this. She is a good kid. . . . I’ve tried to resolve it but I have a lot of anger about it.”

Favoritism creates the most poisonous rivalry of all, agrees Patti McDermott, a Beverly Hills family therapist who wrote the book “Sisters and Brothers: Resolving Your Adult Sibling Relationships” (Lowell House, 1992).

She adds that labeling children creates a perceived favoritism nearly as toxic.

Many parents cast their children in roles to separate them and make them feel more like individuals, McDermott says, “but sometimes this backfires and leads to all the siblings feeling inadequate.”

Roles do create a sense of family order, Greer says. “Our roles help to let us know what is expected of us. Roles become a problem when they stifle our personal growth. It’s like an actress who gets typecast; eventually that role will inhibit her career.”

Mia Hon of Redondo Beach says her older sister Soo always had the role of the responsible “good daughter” and she was the loud tomboy. She remembers beating up her sister until they were teen-agers. Now at 29 and 31, they are roommates.

“She always played the big sister role. She wanted me to be more responsible for the sake of our parents. She would tell me I spent too much on clothes and then borrow all of my clothes. In the Korean culture we have to respect our elders including older siblings.” The Hon sisters still occasionally fight about clothes and responsibilities, but as adult roommates they also respect and defer to each other’s differences.

“All my weaknesses are her strengths and I admire that. She is a great role model,” Mia Hon says.

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Research shows that ties between sisters tend to be closest and between brothers the most rivalrous. Brother-sister relationships are the least likely to clash. Studies find that in our society, as in many others, it is sisters who are the most committed to maintaining family ties.

Deborah T. Gold, an assistant professor and senior fellow at Duke University Medical Center who has written extensively about sibling relationships, believes that parents and society are more likely to compare boys because girls have more lifestyle choices when they grow up. “Rivalry between brothers can grow because comparisons follow males from childhood to the workplace with fewer variations,” she says.

Gold says siblings usually draw apart in young adulthood to establish their own personalities, but by ages 50 to 55 two things happen that usually jump-start the relationship: children start to leave home and parents die.

The most common trigger points of adult rivalries are holiday gatherings, marriages and when parents become ill or die. “It’s those times when we relive our childhood jealousies and resentments,” Greer says. “People go into family occasions with great expectations and the wish for the perfect family. They seize the moment to try and clear up a problem and the result can be very painful.”

If you are striving for change, don’t expect it to happen over a holiday weekend or when the family is in crisis, McDermott says. “It takes time and good intentions on the part of both people. If the rivalry is strong, we are talking about building a trust between two people who have none.”

McDermott recommends that if a sibling throws you a curveball at a family gathering, catch it but don’t throw it back. Instead, acknowledge the problem and set up a better time and place to discuss it.

Experts says preconceptions and existing problems are magnified when parents become ill.

“Parent care is terribly distressing,” Gold says. She finds a family that cannot work together in this situation experiences negative feelings on all levels.

“Parents are really the glue,” McDermott says. “If both parents die and siblings haven’t worked out their differences, they probably won’t maintain their relationship.”

Yet Gold’s research on siblings older than 65 shows that only 10% still harbored hostile feelings between them. Another 10% are apathetic, saying their lives have gone in different directions, and 80% consider their sibling bond a very good, positive and supportive relationship.

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There are ways to improve even the unhappiest sibling relationships. Resolving old rivalries begins with understanding that parents are unlikely to change and that each parent-child relationship is different, even within the same family, McDermott says. Brothers and sisters who step back together and recognize the issues behind their feelings make a good first step toward a new relationship apart from the one that involved their parents. Renewing your sibling ties means talking honestly about how you would like to be treated, she says.

An important component to accomplishing a better bond is setting limits. Greer says siblings must find an appropriate line between what is and isn’t acceptable in their relationship. “Putting boundaries in place allows you to deal with your own guilt and anger,” she says. “If you are uncomfortable with your old roles, step out of them. Let go of your resentment and move forward.”