Over the years, CocoRosie slowly drifted away from their lo-fi roots and toward sounds that emphasize the sophistication of their songs.

On Tales of a GrassWidow, Sierra and Bianca Casady polish away some of the more grating edges of their previous album, Grey Oceans, but these songs aren't all sweetness and light: "After the Afterlife" begins the album with deceptive delicacy before synths take the track in a darker and more mysterious direction.

Indeed, this is some of CocoRosie's most electronic-based music, in large part because the Casadys worked with producer Valgeir Sigurðsson, whose Scandinavian folktronic flair brings out the similarities in the sisters' music to Björk and Múm.

Like those artists, CocoRosie make music that sounds ancient and futuristic at the same time, and the way they contrast and juxtapose those elements are vital to Tales of a GrassWidow's wounded but lovely songs.

There is a lot of grief in this album, particularly on "Child Bride," where Bianca whispers trepidation-filled lyrics as Sierra wails in the distance, or on "Gravediggress," a rueful duet between a young girl (Bianca) and an old woman (Sierra) who may or may not be the same person.

Songs such as these, as well as the surprisingly eldritch "Harmless Monster," where Bianca seems to murmur "I was an angel/Someone's sweet thing" from deep inside a haunted house, showcase not just CocoRosie's increasingly refined sounds but also the more nuanced ways they express their feminism.

Despite GrassWidow's overall somber tone -- which is exemplified beautifully on "Broken Chariot"'s mournful shakuhachi flutes -- the Casadys do allow some hard-earned respites on tracks like the harp-driven "Roots of My Hair" and the equally eerie and catchy "Villain," which proves that the duo can write a pop song (granted, a pop song with harpsichord and violin breaks) when they wish to.

Tales of a GrassWidow may not be as overtly challenging as Grey Oceans, but it offers some of CocoRosie's most focused, accomplished songs yet.