I was diagnosed with CIN III, had the LEEP (loop excision) operation, went on to conceive within a year and carry a huge baby to term. Really, don't worry about fertility.
In my colposcopy, doctor clipped off several nail-tip sixed pieces from the cervix for a biopsy. This is probably what they did to your sister. If CIN III diagnosis is confirmed, your sister needs to have a LEEP.
LEEP does not "burn away bad cells" but cuts away a piece from the cervix that resembles a cone, with a thin hot wire. That is why it is also called "conisation".
Make sure your sister has the LEEP or cold knife surgery, both of which cut out the affected area of the cervix so that it can later be examined, determining whether or not the whole dysplasia has been taken out. If you opt for burning the area with laser or cold treatment, then you can never be sure if the whole dysplasia has been excised.
There is one more issue your sister should probably consider: cervical dysplasia is caused by a virus called HPV. A large percentage of human population carries it, but it does not cause cancer in everyone. However, this means that her partner also has HPV and can give it back to her. His penis also needs to be treated/cleansed of HPV. This is apparently not common practice in UK, but quite important.
Sorry for the very long post, but just thought you would like to have as much information as possible on the subject. Good luck xxx