Why didn't the Missouri prosecutors hold the parents responsible for their daughter's suicide?

If anyone has been following the case where a 13 yr. old girl committed suicide after finding out that the boy she had be chatting with on MySpace was really a girl, her mother, and I believe sister, who intentionally sought her out to bully her. The 13 yr. old girls parents knew that the girl was suicidal. They knew about her mental ailment and sought professional help. The parents allowed this suicidal and mentally ill girl to get no MySpace and now are shocked that she committed suicide. Also, I believe the parents were aware of the feud between their daughter and the other girl. If you need more details just type 13 year old commits suicide Myspace.