Corvah is correct. To my knowledge the fact of being dehydrated cannot in itself cause kidney failure. But could make it worse for someone who already has kidney disease.
What makes you think that your sister already has kidney disease? Is she having any other particular symptom, apart from the dehydration?
When in doubt, get her to go to her primary physician in order for him/her to order the relevant lab work for her. Then, depending on the results of these tests, and if the physician thinks it's necessary, she may get a referral to a nephrologist (kidney doctor). But definitely get her to do this lab work, in order to put your minds at rest, or in case of positive results, to get the proper treatment.
Best of luck,