The City of the Ladies - Matriarchies of Muirin, #1

Anwyn hated Delbhana Siobhan. The worst thing she could imagine was spending time with her. So going to the Delbhana's big party in their new park on the river was a horrible, no-good idea that she didn't get to veto. Even with pastries and her twin brother Cadfael at her side, Anwyn didn't know if she could keep from punching Siobhan right in the nose. Or escape getting punched, either.