A story about a young girl who lives with her aunt and hears ghostly voices and goes in search of her mother's past. She ends up at a school/mental hospital. Her cousin tries to kill her. There's some sort of link to family in a Florida swamp and a hand that's been cut off. I think she's in a band??
Hmm, I couldn't find a cover with a floating dress, but it sounds similar to this book I found on amazon.com called "Gifted Touch (Fingerprints)" by Melinda Metz. It's probably not the right book, but I thought I'd throw it out there. The girl can hear a person's thoughts when she touches their fingerprints. Her mother apparently had the same gift and died in an insane asylum. Someone tries to kill the girl with a bomb. Like I said, probably not the same book.
This is probably not it, but I'll give it a shot: Karmentxu and the Little Ghost by Mariasun Landa. Here's the plot summary from Children's Literature Comprehensive Database: "After a sensitive Basque girl runs away with a ghost to a festival, her aunt places her in a mental hospital, outside of which the ghost watches and waits."
Any leads would be great! Thanks