I've got three ideas:
1) Bury it somewhere. Preferably in a wooded area like a forest. Not only will it be hard to find then, but you can go and read it in an atmospheric surrounding.
If you're going for this option, be sure to place the book in an air-tight container before putting it in the dirt.
2) Hide it in the attic (if your house has one). It'll be safe from your little bro, and you'll feel like one of the Halliwell sisters from the TV show Charmed.
3) A book within a book. First, get a book that's even bigger than your Book of Shadows.
Then cut out a hole inside the pages of the larger book, to stash your Book of Shadows inside. As long as the title of the larger book doesn't draw your brother's attention, you should be safe.
