First of all, juveniles don't go to jail. Second, there is no such crime as "aggravated assault" in California. The term is used in some other states, and can refer to assault with a weapon, or causing serious injury, or in a way that COULD seriously injure the victim. The sentencing range varies depending on the offense, and gets worse if firearms are used.
The goal of juvenile court is supposed to serve the best interests of the child. If she is found to have committed this offense, then the judge is supposed to find the least restrictive placement that will help rehabilitate her. That could be returning her to her parents on probation, possibly with a brief stay in juvenile hall. If that isn't suitable, the judge could send her to a group home or even to the Juvenile Justice division of the state prison system (formerly known as the California Youth Authority) if the offense if a felony.
If it is a MISDEMEANOR charge, she is unlikely to be in juvenile hall much longer.
Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like this site, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.
It is most likely that she will not be spending any time detained in juvenile hall. It is very important that your sister have an attorney that is familiar with juvenile court proceedings. In juvenile court, the idea is to rehabilitate, not to punish. Accordingly, there are many programs that are available, some of which result in the entire dismissal of the case if she does particular things. For a first offense misdemeanor, which it sounds like is the case for your sister, it is likely that she will be able to get the benefits of a 654 motion. Essentially, this will put her on very informal supervision for 6 months and if she does everything she is asked to do, the entire case will be dismissed as if it never happened.
Please make sure that you have an attorney that knows what they are doing in juvenile court.