The Girl Next Door by Mindy Kaplan

Lina is a foreign exchange student working as a nanny for a wealthy family in Washington State. While taking care of four-year-old Sean is taxing, Lina always finds time for her studies--that is, until Alex, the older sister of her charge, returns home for the summer. Lina can't take her eyes off the young woman, and when Alex mentions that she often visits chat rooms on the Internet, Lina decides to seduce her online.