Hi Dottie@76
We cannot give you advice about your sister's move to UC as we do not have all the facts such as did she receive a letter from the DWP about moving over to UC, if she did then she should have Transitional Protection which means she would be no worse off initially.
We would advise your sister to seek advice from her local Welfare Rights Organisation:
advicelocal.uk where they can take all her circumstances into consideration.
You should also seek advice from your local Welfare Rights Organisation.
Gary