Your sister will probably have a Colcoscopy, which is where a special camera is used to look more closely at the cervix. If the doctor feels these cells need to be removed, then he/she will carry out a Loop Diathermy, which is where a cortorising instrument, which looks like a metal wire in a loop shape. This will burn away the abnormal cells.
A biopsy will probably also be taken and your sister will need to return to the clinic about 2 weeks later for the results. I think most abnormal cells caught earlier can be treated successfully. If all is ok, then your sister will be required to have follow up smears every year as opposed to every 3 years.
I hope everything goes well and I think Jade's case was one of the rarer cases, so try not to worry yourselves.