But the pay-off is invaluable: siblings who enjoy - indeed, seek out - each other's company. Companions by blood and by choice.

"The sibling relationship is a dress rehearsal for life," says magazine science writer Jeffrey Kluger, author of (Riverhead Books). "It's where you learn about conflict resolution, where you learn when to stand up for yourself and when it's smarter to stand down. You learn compassion, you learn intimacy, you learn confidence, you learn truth-telling.

"You learn that, left to your own resources, you can work stuff out. That pays dividends in life. But it also pays dividends in the sibling relationship."

Savvy parents know that a conflict-free relationship between siblings is not the same as a close-knit relationship. Swiftly breaking up every spat - or keeping kids sequestered in separate activities to avoid spats in the first place - won't foster much of a bond.

But what will? We turned to the experts for advice on cultivating sibling bonds that will withstand the wear and tear of time. Three themes emerged.

Model respect. "When parents model how to talk and how to listen, children learn how to communicate with siblings," says Isha Williams, licensed marriage and family therapist and professor at Adler University in Chicago.

"That means setting the tone early and maintaining it as everyone gets older; that you're going to have conflicts, you're going to get angry, but it's never acceptable to hurt each other. It's no more acceptable to throw things at your siblings than it is to throw things at your parents."

Williams says some families opt for regular family meetings to air grievances, share news and otherwise stay connected on what's working and what isn't. That can be a useful time to model active listening, taking turns and not interrupting - skills that will benefit every relationship your children have, including the one they share with each other.

"People will hurt your feelings and you can't always just take your ball and leave," Williams says. "Teaching your children what to do and say when they feel angry is the basis of them being able to engage with each other, as well as with friends and co-workers and other people in their lives."

Don't intervene. That modelling should be accompanied by a willingness to let your children work through most of their disputes, experts say. "Unless there's a real risk there's going to be bloodshed or there's bullying going on - one older or much bigger sibling is picking on another one - parents should not break up fights," Kluger says.

For starters, there are often too many to stay on top of.

Beyond that, children need to learn to resolve conflicts peacefully and effectively. What better place to hone that skill than at home, surrounded by people who are prone to give you second, third and fourth chances?

"It's very powerful to learn, A: there are ways to solve fights," Kluger says. "And B: how very good it feels when a fight is over."

Knippen says she calibrated her intervention style when her children were young, thanks mostly to , Adele Faber and Elaine Mazlish's 1987 parenting manual that encourages parents to recuse themselves from most of their children's disputes.

"I felt like we would have a more peaceful family if I changed the way I was handling it," Knippen says. "It felt more respectful - from me to them and for them to each other - for me to back away."

Which isn't the same as stepping out altogether.

It's important, Williams says, to understand each child's capabilities within the family dynamic. "You sometimes see children who are four or five with really good verbal skills, and parents think they're capable of behaving the way their older child is behaving," she says. "But a four-year-old and a seven-year-old are at very different levels, from what they're going to want to do at playtime to how they respond to their emotions."

Be aware of patterns: one child chooses all the activities, one child takes all the turns, one child frequently dissolves into tears. Stepping in with a reminder to follow the golden rule and a quick "I trust you to solve this one together" can keep bad habits from cementing and can, hopefully, stave off lifelong resentments.

Encourage togetherness. From the time Knippen's children were toddlers, she and her husband chose toys their children could play with together. "We consciously chose Playmobil, for example," she says. "That allowed them to build castles and oceans and pirate ships, and the pirate ships would attack the village. And of course there would be spats, but they resolved them and became closer because they knew they could work through stuff."

Looking back, Knippen says, she may have parented her kids in response to her own childhood. "I'm the oldest of three, and my brother and sister are both deaf," Knippen says.

"We grew up very close, but in a nontraditional way. I never went to school with my brother and sister. They were mainstreamed into a school some distance away and I felt like that affected our ability to relate to each other. We didn't have the same friends. We didn't have the same teachers."

So all three of her children were asked to attend each other's recitals and big events - a request that inspired a bit of pushback, she says. "But we really tried to set it up as an important idea that you support each other."

Siblings who are encouraged to support each other's endeavours often get to know one another on a more multifaceted level, Kluger says.

"It's important to see what your sibling is doing and experience the joy with which they're doing it," he says. "You may not acknowledge it at the time - I used to glower when my younger brother Bruce would be in a play, because he was so good. But by the time I got to college, I'd look back in admiration and wonder."

A huge part of adoring each other, after all, is knowing each other: hobbies and habits, tics and talents, passions and pet peeves. "To a very large extent that happens naturally," Kluger says. "You live under the same roof and share the same toys and meals and parents and you're going to know each other really, really well. But what's in your heart? How do you look when you're happy? How do you feel when you're doing what you love best? That's the other side of your siblings."

Last summer, Knippen's 19- and 16-year-old daughters travelled together to Norway, without their parents, to visit a young woman who lived with the Knippens as an exchange student. Both girls cherished the trip deeply, and Knippen delights in the fact that they chose each other as travel companions.

Asked what's her favourite part about watching her mostly grown children interact, Knippen doesn't skip a beat.

"The laughter," she says. "I laugh until I cry."

That's a pretty good pay-off.