As per your history it appears to be chronic Febrile illness. The doctors must have advised you for blood reports. If the blood reports are non suggestive of any disease then we have to go for bone marrow examination to find the cause of fever
of unknown origin. You can discuss this things with your doctor. Also as she is not gaining weight
it is important to keep a check on the diet
that she is taking your doctor will help you in this also.