Your sister needs to contact a family law attorney to have him or her prepare the temporary guardianship paperwork that will be approved by a judge and filed in the juvenile court in the county in Georgia in which your sister and the children currently reside. Your sister needs to make sure that the papers specify the specific length of time of the temporary guardianship over the children. Since everyone it seems everyone is in agreement with the guardianship, it should not be costly, she just needs to make sure it is done properly.
This is a general interest posting on a public website, regarding general legal concepts. This posting is not, and should not be interpreted to be legal advice to anyone, or to create an attorney client relationship between the poster and anyone.