Termination of a probationary employee

Dear PAO,

My sister was hired as a probationary employee last December. Unfortunately, our family went through a series of problems early this year and it affected every one of us, including my sister's performance at work. She was notified by their Human Resource Department that her contract will expire on the last day of her probationary period. She was told that the company will not regularize her because her performance was graded as "poor." Is this kind of termination valid? Shouldn't there be a just cause for the termination of her employment? Please advise.