SOLVED. Middle grade fantasy about a girl and her brother who have to save their castle [S]

[Amanda]

My daughter and I read this book about three years ago. I think it was written within the past ten years. In the beginning the parents are away for some reason, and the brother is trying to come up with a magic potion of some sort. I think it's someone's birthday--possibly the girl's. Or their mother's. There's a magic book involved--possibly a spell book. At some point the girl, disguised as a peasant (or something of that sort), sneaks into another castle to get help. At the end of the book the castle is under siege, and the girl and her brother have to save it.

[Andria]

Possibly Igraine the Brave by Cornelia Funke?

[Amanda]

That's it. Thank you!