This immune reaction causes some of the body's cells to release histamine and other substances that cause the allergic symptoms. Many different organ systems can be affected, producing symptoms such as hives, itching, rashes, swelling of the face, hands, or feet, abdominal pain, nausea, vomiting, diarrhea, and asthma. As far as gluten specifically, it is a protein found in wheat, oats, barley, and rye. As defined separately from allergies, there is an intestinal disease that is affected by gluten intake. Celiac Sprue symptoms can range from being life-threatening to manifesting as anemia or low bone density from malabsorption.
Treatment includes a diet free of wheat, oats, barley, and rye.
It sounds like your sister may have been diagnosed with Celiac Sprue, and, if so, would need to work with her physician to determine an appropriate diet and course of action to minimize the effects of her low bone density. It is not unusual for a doctor to overlook the possibility of Celiac Sprue, as it is a fairly rare disease. She simply has to take the time now to see what changes she can make to improve her health for the future.