For the second time in a weekend, Cindy found herself banishing her two daughters to different corners of their home. She wondered where she had gone wrong as a mother - the girls' squabbles, screaming and general nastiness had escalated into physical slapping and scratching.
"It was a classic case of sibling rivalry, but I thought that only happened to babies and toddlers," she says. "What is so upsetting is that my girls aren't little any more - they're 13 and 16. They used to be such good friends, but instead of sharing clothes and secrets, all they do now is fight. Our house is like a war zone."
Most people, like Cindy, think of sibling rivalry as something that happens when children are small, but experts say it can happen at any age, and the teen years can be particularly difficult. Feelings of competition, inferiority, jealousy or simple personality differences can lead to disharmony at home.
"Every family has sibling rivalry to some extent," says Dr John Shanahan, a developmental psychologist at HK Central Health. "Factors such as the number of children and their ages really have an impact. For instance, there is the three-year-old who wants to poke, pinch or hurt the new baby; the eight-year-old who wants to wear make-up like her 14-year-old sister, or the 16-year-old twins who fight each other on the rugby pitch."
Although Shanahan mostly encounters sibling rivalry in younger children, he says conflict in teen years is generally due to feelings of inferiority, where one sibling seems better at everything: exams, sports and dealing with parents and peers.
Parenting coach Katherine Sellery agrees. Left unchecked, she says, feelings of inferiority can damage a relationship beyond repair. Many children have insecurities and these can be exacerbated when a child is not good at something but their sibling excels at it.
"What children don't realise is the feelings are there not because of a sibling's success but because of their own need for competency; they make the mistake of blaming the other person for how they feel," Sellery says. "This emotional immaturity is really at the heart of the pain because they go about trying to make the sibling change their behaviour or get them into trouble under the false assumption this will somehow make their own feelings shift."
In extreme cases, experts say, this can cause a child to become withdrawn, unmotivated, irritable and aggressive.
Zoe and her brother, Sam, are aged 23 and 25, but they've only just started to get along.
Their rivalry was at its peak during their teenage years, she recalls. "Sam did everything he could to be nasty and difficult. I think he wanted to be an only child and I was just a thorn in his side from day one. For example, we had to share a room when we were little, but he wouldn't let me in unless I paid him. Once I hit my teens, I fought back and made it my mission to outshine Sam in every aspect of life. It got pretty intense."
So, what can exasperated parents do when teenage siblings are constantly at each other's throats?
Experts agree parents can help in several ways: among them, by stepping in to mediate and also by giving children the tools to cope with their own feelings.
"I would advocate moderation," says Shanahan. "Parents should allow some healthy rivalry because this can teach conflict resolution, competition and sharing. But they need to step in if it is becoming too unhealthy. In the teen years, parents need to shift from being a manager to being more of a consultant."
His advice for parents is to teach problem-solving strategies alongside effective communication.
"Family time is important to encourage siblings to see the family as a team rather than a group of individuals. Encourage team-building games or activities and help them find common characteristics or interests, as this will strengthen their relationship," he says.
A "sibling night" when the youngsters can run the evening - choose a menu, cook the meal and find an activity in which everyone in the family can participate - can be a way to break down barriers, Shanahan says. This will encourage co-operation and help the youngsters discover that their talents may lie in different areas.
"Children can address the situation by understanding there are some things they are better at than their siblings, and some things they are not as good at."
Sellery says any form of sibling rivalry is damaging to a relationship, unless a parent quickly intervenes and uses the right conflict resolution skills so that learning can take place.
As she sees it, a parent should simply act as impartial mediator, someone who doesn't take sides but instead acknowledges how each teenager feels and makes sure the children are also aware of each other's feelings.
"By saying to each child what the other is feeling and needing, without taking sides and making one right and the other wrong, you become a trusted facilitator," Sellery says. "It also means there is no benefit to running to you and 'tattling', as this doesn't get the other child in trouble. The parent only ever acts to get the feelings of both sides expressed. There is no judgment attached."
Importantly, parents should be aware they can often exacerbate an already touchy situation, Sellery says. She cites clinical psychologist Dr Thomas Gordon, founder of Parent Effectiveness Training, who says a parental discipline approach based on reward and punishment only creates an environment of competition and actually fosters sibling rivalry. "Instead, use a guidance approach and abandon rewards and punishments. If you do, sibling rivalry will diminish."
Zoe and Sam have spent a lot of time over the past few years talking about the issues they had as teenagers. Much of the frustration, Zoe says, was due to her own difficulties with sharing their parents' attention. This was particularly because her brother did not do as well in school and, therefore, in her eyes, was far less deserving of their time. By working through these feelings with Sam, the two have started to build a positive bond.
"Our friendship as adults has grown from honest discussions about the rivalry we shared as children," Zoe says. "Understanding each other's point of view and experience of our parent's attention has thankfully brought us closer."