So, here's a bit of info about the girls in the page. Moraine has two younger sisters. There's Maud, also called Maudie, who is ten years younger than Moraine. The youngest of the trio of girls is Mathilde, nicknamed Hildie. She's fifteen years younger than Moraine. Moraine is currently twenty-seven in A Year and a Day, which means Maud is seventeen and Mathilde is twelve.
As for their behavior here... hey, just because one's a princess doesn't mean there's a lack of sibling rivalry.