Fablehaven
A book series that's not that great, but it's about a brother and sister who get stuck on either the good or bad side of magic. They still have to be a team, even though they're not getting along. The sequel series is called Dragonwatch.
'My sister is the light side, and I'm the dark side. We're like enemies.'
'This book is the worst, but I still read it because I have to.'
'I can't believe I still have to team up with my annoying brother.'