Few families are immune to some sibling rivalry, but what happens when it becomes bullying and what can parents do about it?
It's an issue that Corinna Tucker of the University of New Hampshire feels passionately about. As the lead author of a new study for the American journal Pediatrics, she found that almost a third of children said they had been victim to sibling aggression during the past year. That's more than the number of kids who have been bullied at school, which studies show affects up to a quarter of children.
The aggression can take many forms, says Tucker, including theft, psychological abuse and physical assault.
"It's not just younger children," she adds. "Our study questioned both children and adolescents. This is significant because many parents assume sibling fights are over by the teenage years."
"I'm so scared of my brother," one 15-year-old girl recently told ChildLine. "He pushes me, shouts at me and sometimes even hits me. Mum and Dad don't do anything to stop it. I cry almost every night and am so angry with everyone. Sometimes I want to disappear."
Tucker found that even one act of aggression in a whole year can lead to mental distress, although – as you'd expect - the more incidents of aggression, the greater the link. "By mental distress, I mean depression, anger and anxiety," she explains.
Even mild incidents can have an impact. "This finding was something that took people by surprise – the fact that even mild experiences, such as being called names or being on the receiving end of mean words, can leave young people feeling mentally distressed."
If you think about it, the home is the perfect breeding ground for bullying, says Tucker. Not only do children live together for a long time, but there's usually an imbalance intellectually and in terms of physical power. Plus, siblings often experience jealousy, as well as often competing for parents' attention. The relationship is emotionally intense too and is one of those where you can love and hate someone all at the same time.
In fact, other studies show that siblings can be both a bully and a victim, probably helping to explain why it's often not picked up by parents.
Other reasons it remains a well hidden problem is that parents often seem to expect their children to fight with one another, says Tucker.
"Many parents think it's OK for one of their children to hit a sibling on the head, but not a friend. Why is that? It's like parents have these different norms of acceptability and that's what I'd like to see change."
What's more, she says, some parents actually think it's healthy when they see their children failing to get on.
"You hear parents say things like, 'Sibling relationships are the first place kids learn how to get along with people' and 'Not getting on with his brother will certainly teach him how to handle difficult situations in life.' But what they're missing is that many of these kids are suffering mental distress as a result."
Although Tucker's study didn't look at the long-term effects of sibling bullying, anecdotal evidence suggests they can be serious and life-long, with many people blaming their parents for ignoring the issue.
"I know my mum and dad saw my sister constantly saying the most awful things to me, things that exacerbated my eating problems during my adolescence, but I think they felt overwhelmed by it and chose to ignore it," says Rachel, 35.
"I've had to go into therapy because of the impact it's had and I can't help feeling most angry of all at my parents, who should have stepped in and protected me."
But with few sibling groups getting on all the time, the line between normal discord and bullying is hard to spot for parents.
Watch for patterns developing over time and be sure to talk to your child if you feel he or she may be at risk, advises Tucker. If they won't open up to you, find someone else, perhaps a counsellor.
Never belittle any bullying behaviour, she adds. Learning to recognise that it's just as serious as school bullying is critical.
"Don't have different rules for inside and outside the home. It's that simple."
Sue Minto, head of ChildLIne, agrees. "All bullying should be taken seriously and it is important to recognise the significant impact it can have on children. To be bullied at home by a sibling also means that for the child being bullied, home may not feel like a safe place. It could be that a parent would underestimate the impact on their child and it is important that we recognise that bullying by a sibling can be every bit as damaging as bullying outside of the home."
Work on prevention too, says Tucker. "All parents owe it to their kids to focus on conflict resolution. I mean, people are quick toteach their kids athletic and academic skills, but not relationship skills and even when they do, this sibling area is often ignored. You couldn't go to school or to work and just hit someone, so why is it OK at home? Teach your children how to resolve fights through mutual problem solving and respectful talking."
There is often a general assumption that kids need to sort themselves out in the home, says Jeremy Todd, head of Family Lives, the parenting support charity "But that's really unhelpful."
Todd adds that parents must be consistent in how they respond to their different children's needs in order to try and avoid, or at least minimise, jealousy.
"Each child will benefit from different types of activities and attention. This also helps in showing that no one person has a monopoly on your attention and is more important or able to gain more attention or manipulate you."
If a parent instinctively feels something is wrong in their children's relationship, act on it, he advises. "If you don't, this can be a damaging and lifelong experience, not just for the victim but the bully too."
Indeed, Rachel, 39, who reports that both verbal and physical agression dominated her relationship with her brother right up until her mid-teens, says, "My brother was physically aggressive, but I more than made up for it with my nasty remarks – remarks that I believe have helped lead him into a life of drug problems. He doesn't speak to me now and I can't seem to forgive myself. Sometimes it eats me up inside."
Parents who need help with sibling bullying should visit Family Lives or call their parenting helpline on 0808 800 2222