Just your typical 'move into an old creepy house, weird stuff starts happening, one of the kids gets blamed, and then they find a book that explains the existence of fairies/faeries/fey which solves their problems... while at the same time causing many more problems'.
Happens every day.
Also, I'm no expert in the sport of fencing, but I'm pretty sure that they don't call the weapon a sword, and someone who is serious about fencing, as Mallory apparently is, would know that.
I think that this book reads a little young, also. Not just for the length, which is REALLY short, but also in the style and feel of the writing. The characters are 9 year old twins and a 13 year old older sister, but if I had to peg an age range for the target audience, I'd go with 6 or 7 year olds.
Still, this is probably a fun and cute story for kids. I'm just less than impressed by it, considering that I'm a crotchety 30-something cat lady.