Yes, any court action against your sister for debts should be in a court that is local to her. She can apply to have the CCJ overturned.
The first step to removing inaccuracies is to contact the creditor involved. If the information is wrong, the creditor should notify the credit reference agencies within 28 days. If a creditor refuses to do this, your sister can contact the credit reference agency(s) and ask them to review their files, but she would need to do this with each agency. For the CCJ, you may need to contact the Northampton court to apply for the judgement to be removed.
If the above does not produce a satisfactory result, your sister may write a "Notice of Correction" of up to 200 words and she should send this to the credit reference agency(s) and they should attach this to her file.