Thanks for the query.
You have not mentioned your sister's age. Also does she have a history of Diabetes or high blood pressure? Also what is her creatinine
A GFR of less than 60 for more than 3 months can be a sign of Chronic kidney disease
A person suspected to have Chronic kidney disease needs to undergo further blood tests, urine exam and ultrasound
exam of the kidneys and bladder to confirm the diagnosis and determine the cause.
Your sister needs to see a nephrologist
Hope this helps