I think this is a great idea to have Victoire and Teddy at odds with each other in this story. It adds some spice to the beginning, and also makes me wonder if you will keep them this way, or if they will morph into something different later on. Thoughts...
Your dialogue was written very lively-like. I could hear all the characters in my head as they yammered at each other and annoyed the living daylights out of each other as well. It seemed realistic that way, how the cousins interacted at the obligatory family gatherings. I could see how some of them were tired of each other, and others were more inclined to like the gatherings. Nice portrayal of the family event.
One thing I wasn't sure about was Victoire's mother not checking ALL her pockets. If I had a daughter whose pockets needed checking, I'd check ALL of them, and demand to see the invisible pockets as well. If you've got a smart kid, you've got to keep a few steps ahead of them. Just saying...
It was a little sad that poor Victoire felt like her birthday wasn't the best. It didn't sound very pleasant, but I like that she took it in stride. I'm not sure how I would have felt if she'd been overly dramatic about it . I'm glad she wasn't, because that would have put me off the character. I'd not have been so sympathetic. I think your choice to have her accept it as it was made her more relatable and also gave me more sympathetic feels.
So all in all, this was a typical family gathering, and I got a good feel for the characters. I wonder what's going to happen next?