You can't get her out without paying the whole amount no, but she could ask for reconsideration of bail to see if the judge will change his/her mind, by writing a letter to the court, asking her attorney to request a hearing on her behalf, or just waiting until her next court date and asking that judge. The first two will likely be unsuccessful however, unless cash-only was ordered by mistake. Waiting until the next court date is probably the best option, as there is usually a different judge hearing the case .
It would be fairly uncommon for someone with only a suspended license charge to have cash-only bail set. For someone with a bit of history though, that might make sense. As the other attorney pointed out, you might do best to address this at the next hearing. Also, your sister really needs an attorney. If she cannot afford one, an attorney will be appointed for her. Her attorney would be in a better position to ask to have bail reconsidered, as the attorney would be able to consider other facts that you may not have.