Joanna Fortune: My young daughters argue all the time

"Both are in the middle childhood phase (eight to 12 years), meaning they are competitors within the same developmental stage, which can lead to the type of tension you describe."

I can’t remember a time when they were at ease in each other’s company

My daughters, aged 11 and nine, constantly argue. They have very different personalities — one is messy and talkative, while the other is tidy and quiet. 

I can’t remember a time when they were at ease in each other’s company. I’m already dreading the teenage years. What can I do to help them get along?