Hello,
The various possibilities which present with such pain in the lower left abdomen are Crohn’s disease, bowel obstruction,
ovarian cysts and pelvic inflammatory disease, irritable bowel syndrome , kidney obstruction or pyelonephritis, psoas abscess and volvolus (twisting of the sigmoid part of the large bowel on itself).
Investigations like ultrasound abdomen, CT abdomen and colonoscopy are required to confirm a diagnosis. your sister should get these investigations done.