You could get some advice from the GMC as to how to go about things eg whether you should address it direct with the GP surgery or hospital (was it a GP or hospital specialist?).
It may be worth asking about the circumstances and how it happened such as was there a mistake that it wasn't spotted at X-Ray or the doctor so you have a better idea of what went wrong.
Could it be advances in screening and interpreting x-rays which mean it has only just been picked up now?
You could find out if it would have made any difference if she had known eg quality of life, medication, lifestyle choice, anticiaption of life expectantcy.
GMC -
http://www.gmc-uk.org/
For you and your sister, get an appointment with your GP to discuss your concerns and make sure you have the proper tests or referrals with follow ups and repeats if necessary.