When Bianca Casady showed up on the Parisian doorstep of her older sister Sierra in 2003, her plan was to visit the sister she hadn’t seen in nearly a decade, and then continue to travel the world. Instead, the sisters essentially barricaded themselves in Sierra’s bathroom for two solid months to make music. When they emerged, they had fused their childhood nicknames into CocoRosie and became a full-fledged sister act.

Their style of music, sometimes classified as psych-folk or indie rock, included Bianca’s darkly cute baby voice howling over music made from toys, electronic equipment, beat boxes and more traditional instruments like guitars and harps. The duo became a starting point for a new breed of baby-voiced rockers.

Like Joanna Newsom, Satomi Matsuzaki of Deerhoof, and possibly Björk, CocoRosie can come off as stunningly beautiful and deep when done right. On the other hand, it’s grating to the nerves when done poorly.

“The Adventures of Ghosthorse and Stillborn,” the newest album from the sisters, reveals a little more of their hip-hop influences while still maintaining a mystical, dreamlike quality.

“Rainbowarriors,” the stand-out opening track laced with Bianca’s hauntingly precious vocal styles, is a glittery gold star that peels back to flash its hip-hop soul.

The album plays out much as the childish title would imply – like a nursery rhyme on acid or a wonderful dream suddenly turned foreboding. The sister’s mix their distinct penchant for quirky sound samples (horses whinny, toys bleep and babies whimper) perfectly with their childlike vocals.

Although this might make CocoRosie sound immature, their lyrical content is anything but. On “Promise,” where a boom-click-clack beat and warped, wavy echoes offset Bianca’s saccharine, sweet and innocent voice, a surface listen might make it sound like an urban lullaby. But, like the rest of “The Adventures,” a closer listen to the songs’ words makes it clear that the lyrical content is about sex, not the ABCs. Bianca sings, “Promise me you’ll cherish / This tarnished offering / And if you take me inside / And you give me a place to hide and cry / I’ll bathe you in the crystal light / That sleeps between my thighs.”

The album, like on “Promise,” comes off as straddling the line between maturity and youth. CocoRosie has figured out how to replicate through their lyrics the stark realities of our world while still expressing a nostalgic longing for the simple, innocent days of childhood.

For most of “The Adventures,” CocoRosie induces shivers and promotes repeat-playing of the album.

However, on the album’s only real disappointment, “Japan,” CocoRosie are as annoying and headache-inducing as a toddler throwing a temper tantrum. “Japan” features looping baby-talk over a soaring timpani ballad with operatic background vocals. While novel at first and with perhaps the best example of why listening to Bianca’s lyrics is essential to digesting CocoRosie, the baby-voice is entirely too taxing.

While the seamless integration of soaring, epic ballads and barely-twitching, eerie whispers on “The Adventures of Ghosthorse and Stillborn” signal a continued departure from CocoRosie’s previous albums, it also shows that, even as they persist in experimenting with sounds and musical styles, they are clearly becoming more comfortable in their roles as musicians.