I have a daughter of 9 and a son of 13 and to say they don't get on would be understatement of the year. They constantly nark at each other and today a spat over TV watching ended up with my 9 year old DD braining her big brother by throwing a boot at his head.
They have got along very badly for the last couple of years, made worse by my son going into cool teenager mode. He picks on her for being babyish or being annoying, and she plays up to it. I wonder also if it's complicated by my daughter having specific medical issues - she has severe food allergies plus asthma and eczema, so we have always had to be very careful with her. Unfortunately, she now has explosive tantrums that are at the absolute worst with her brother.
Any ideas on how to improve things would be really welcome. I'm an only child myself so I have no idea what's normal - but this just seems really, really bad.