Hello hun
Unfortunately they can just sack people like that if they are temping through an agency - ive had to sack people for no reason on hundreds of occassions and its not easy
Usually the company is asked by the agency to give temp staff either a week or a months notice (depending on how long they have been temping) so that the person is given time to find alternative work. Most companies tend not to do this - it is not a legal requirement and whereas some companies will give notice as a 'goodwill gesture' ive found that most do not.
Sadly, through my experience ive noticed that companies who are having financial difficulties or are struggling somewhat, often take on a number of temp staff for this reason - if things do start to go wrong for the company, it is very very easy to make cuts.
Im sure your sister hasnt done anything wrong. The amount of times my temps have been told by the company they are being sacked for unsatisfactory work/inadequate performance etc and then the company have called me and told me they are cutting back on staff due to financial probs/budgets etc is unvbelievable. These poor people think they have done something wrong when in fact its just the company who havent got the balls to tell them the real situation.
Hope this helps to some extent
xx