Wildwood Dancing by Juliet Marillier
This story takes place in Romania and tells of five sisters who travel through a portal every full moon to dance in the Realm of the Faerie. Jena, is the second oldest of her father's daughters, Tatiana, Iulia, Paula and Stela. The two eldest sisters, Tatiana and Jena, take it upon themselves to maintain their father's household while he is away for the winter due to illness. This does not prove easy as their cousin, Cezar, seeks to intensify his grip over the household and the girls. Jena's problems become even larger when her elder sister, Tatiana, falls in love with a creature of the faerie realms, one of the Night People, a vampire. With the help of her sisters and her best friend, a frog, can Jena prevail over Cezar and sinister forces within the Faerie Realm?
Also by Juliet Marillier:
Cybele's Secret is a companion novel to Wildwood Dancing, taking place six years later. The story follows the youngest sister, Paula, as she travels with her father to Istanbul in search of the Cybele's Gift, a pagan artifact which threatens the Muslim political powers with its very existence. Pick up this novel if you liked Wildwood Dancing, it will not disappoint!