Thank you for the query.
This strange things can be undigested food parts, parasites or blood. Undigested food parts can appear due to pancreas
issues, inflammatory bowels disease, coeliac disease, food allergy
.Blood in stool
can be caused by hemorrhoids (in such case it appears on the surface of the stool and toilet paper, itching around the anus can be present), inflammatory bowels disease, bacterial infection.
I suggest your sister to have diagnostics done. Stool tests, stool culture test and colonoscopy
should be done.
Hope this will help.