I have younger brothers and sisters and by and large they are very well behaved and don't get away with a lot, however Kali ( one of the twins) can tantrum to world class standard if she wants to and being little it's to be expected sometimes. It's not worth getting upset about, little kids are sometimes horrid, often in fact, she told my mum's boyfriend that she hated him and he was ' big and ugly' because she couldn't have her own way after he'd taken her pony trekking and out for the day, so don't fall out with her parents over it and just try to forget it. He did and now things are mostly nice between them. I wouldn't have her sleeping over though if she unfettles your peace and calm because you can't expect her parents to rear her the way you would, she's not your child.