What is the main conflict in High Wizardry by Diane Duane?

High Wizardry by Diane Duane is a fantasy that is the third book in the Young Wizards series, preceded by Wizards Abroad and Deep Wizardry. In this third of the continuing series, Nita's younger sister Dairine discovers her sister's wizard's manual and looks through it, reading the Oath that her sister and Kit took. It is not long afterward that she discovers she also possesses the power of Wizardry. Nita has even informed her parents of the truth and though they took the news relatively well, now every time Nita leaves the house to spend time with Kit, they worry that she is on an assignment and will be in danger.