Your sister is suffering from pica, which is the persistent eating of non-nutritive substances at an age when this behaviour is inappropriate. It is commonly seen in children and when present in adults, may suggest a developmental disability. It can also be present in young pregnant women in their first pregnancy. The cause for this is unknown and a lot of theories abound that implicate cultural, socio-economic, psychodynamic and other factors. Some nutritional deficiencies (iron, vitamins, zinc, calcium etc.) may be associated with this habit. It is not clear whether iron deficiency leads to pica or vice-versa. Hunger or malnutrition may sometimes lead to it. Consultation with a psychologist or psychiatrist is necessary, as behavioural therapy has been seen to be effective in such cases.