PHOENIX (Court TV) — A couple faces life behind bars after their nine- and seven-year-old daughters told authorities about the murder of another sibling.

Christine Tuong died of a punctured liver, but how she sustained such massive body trauma is in dispute. Prosecutors claim Tung and Minh Tuong hit their daughter 27 times with a belt and left her to die in their own house.

The case seemed unfounded until Christine's two sisters came forward and told police that their parents frequently beat them and forced them to lie to investigators. Subsequent medical examinations have also shown that all three daughters have been molested.

The Tuongs deny hurting their children and claim not to know why Christine died.

"We are still wondering what happened. We don't know," Mr. Tuong told The Arizona Republic. "We raised our kids in a disciplined way. If you see our kids and talk to them, you know they are raised with discipline."

Mrs. Tuong speculated to the same paper that Christine suffered an injury while playing with friends.

"There was, like scratches and scrapes. I asked (what happened), but she didn't tell me," she said. "I thought I could call (the doctor) the next morning. I thought she just needed a good rest."

The Tuongs' trial, scheduled to last four to six weeks, will begin January 3.