Your sister was on probation for 6 months and shel left job during this 6 months and her services were not confirmed in writting.
During probation employee and employer has right terminate services without notice by either side.
No notice is required during probation. Probation means employees on trial therfore continuity of job is not surity.
I believe company will not waste time and energy chasing your sister if theyare wise and good organization because they did neither confirm services not acknowledge continuation of her services.
30th August 2008 From Saudi Arabia