The Embroidered Sunset — Joan Aiken

The two old ladies had lived there together for so long that even their friends found it difficult to tell them apart. One thing was certain, however: one of them was dead (murdered?) and the other had disappeared.

To unravel the tangles for herself and her Under Wilbie, young Lucy Culpepper decided to find her old Aunt Fennel -- was she the one who vanished, or was she the one who died? -- and as if the floodgates of evil were suddenly opened, Lucy discovered that someone urgently wanted her dead....