High Wizardry — Diane Duane

Young wizards Nita and Kit face their most terrifying challenge yet: Nita's bratty little sister, Dairine. Not only is Dairine far too smart for a ten-year-old, she also has recently become a wizard, and worse yet, a wizard with almost limitless power. When Dairine's computerized wizard's manual glibly sends her off on her novice adventure--her Ordeal--Kit and Nita end up chasing her across the galaxy, trying to catch up with Dairine before she gets into trouble so deep that not even her brains can rescue her.