"Karen's New Year" (Baby-Sitters Little Sister #14) by Ann M Martin

It's fun to be a babysitter' Little Sister!

For New Year's Eve, Karen thinks everyone should make a promise. Hannie is going to stop biting her nails. Kristy promises not to talk to her boyfriend on the phone so much. And Karen makes the most promises of all ― nine!

But pretty soon, everyone starts breaking their promises. And her brothers and sisters are calling Karen a spy!