If you think she is truly suffering than she needs to be re evaluated. She may be resistant to the particular antibiotic she was given.
How Long Does It Take, While On Medications, To Get Rid Of A Kidney Infection And Start Feeling Better?
UTI infection can be treated within 5-7 days. But your sister is more prone to get reinfection because of kidney stones. It will be good to get remove these stones by surgery and then this UTI problem will be solved.