From Viz:
Aya and her twin brother Aki thought they were going to a celebration of their sixteenth birthday at their grandfather's home but the funeral-like atmosphere tips them off that something's not right. Their "birthday present" turns out to be a mummified hand. The power of which forces an awakening within Aya and painful wounds all over Aki's body!
Grandfather Mikage announces that Aki will be heir to the Mikage fortune and Aya must die! But Aya has allies in the athletic cook and martial artist Yuhi and the attractive, mysterious Toya. But can even two handsome and resourceful guys save Aya when it's her own power that's out of control?

Ayashi no Ceres Gaiden is included in 6th volume of Fushigi Yuugi - Genbu Kaiden.

Note: Won the Shogakukan Manga Award for Shoujo in 1998.