She has abundant legal options enumerated hereunder.
1. She may file a FIR against her step-grand mother and the children of her grand mother for subjecting her to physical torture. All of them may be arrested and put behind bars for the felony committed by them. Since she is the sufferer her own statement is sufficient for the case to be lodged against all of them and them to be arrested in pursuance of police investigation.
2. If police does not lodge FIR then she can file the criminal case directly in the court whereupon the court, after recording a finding of the commission of offence by her step-grandmother and her family, order all of them to appear in the court and face the criminal proceedings.
3. Although the case can be lodged by her at her own place, but this is subject to how long back the offence was committed, domicile of all the abusers and the nature of injuries, if any, which had befallen to your step-daughter on account of physical abuse meted out to her.
4. In addition to to above, she can file a case against all of them seeking heavy monetary damages for the physical abuse sustained by her.
5. Consult personally with a lawyer before you move to the court as any weak link in the legal strategy so devised in such cases may offer an escape route to the accused.