My little sister is 16 will be 17 in November ( she is my step sister) she is living with her mother in Georgia, already has been raped, she is not take care of at all, she is doing bad in school , and is scared to death to be in the same house with her step-dad. Now, she has three other brother all three decided to move in with my mom and their dad. Now she wants to move in with them, they live in Texas. What age can she decide? Or do you know where I can find this kind of information?

The standard is age 12, maybe younger, depending on the child. It sounds like she needs a child advocate to intervene in her situation, which the courts can appoint.

There is no age that she can "decide" the non custodial parent will have to file for custody in court, then the child can testify to what they want, they judge will take what they want into consideration but will look at other factors.

What has the police done about the rape when she reported it