Either she does not eat well or there is increased metabolism as
in hyperthyroid states, HIV, cancer etc. Poor absorption as
in irritable bowel syndromes and malabsorption syndromes too is possible. Malabsorption syndromes are seen in intestinal tuberculosis, HIV, tropical sprue, parasites in stool and Whipple’s Disease.
Do discuss this with your step-daughter's doctor and get her examined. Take care!