I'd be real careful on this one. Sadly, there are legal ramifications for you since the DA refused to press charges if you do say something. At the same time, there are moral obligations. Phrase things very very carefully. Do not state anything as fact, just as rumor.
Does the 9 YO know what has happened to her sisters yet? If not, this is something else that would have to be factored in when telling the parents. Would they tell their daughter why she can't go? If so, there is no way it won't get back to yours.
People are going to wonder why this little one is allowed to go to someone you suspect of molesting little girls. They're not going to understand courts, etc.
Sadly, you're on very soft ground on this one. There are no, right good answers for your family. There is simply the moral obligation to protect an innocent. I'm sorry.