Wildwood by Colin Meloy, illustrated by Carson Ellis - a MUST READ.
Amulet by Kazu Kibuishi. This graphic novel series is another MUST READ.
Gods of Manhattan by Scott Mebus (Ok, another MUST READ)
Fablehaven by Brandon Mull
100 Cupboards by ND Wilson
Sisters Grimm by Michael Buckley
The Secrets of the Immortal Nicholas Flamel by Michael Scott
Cold Cereal by Adam Rex
The Spindlers by Lauren Oliver
The Blackhope Enigma by Teresa Flavin (two-book series)
Seven Sorcerers by Caro King. A top-ten favorite of mine, also atwo-book series)
Breadcrumbs by Anne Ursu (best friends, not siblings, but SO good!)
The Little Secret by Kate Saunders (again, friends, not siblings, but sooo good)
The Paradise Trap by Catherine Jinks