I am sorry to hear about your mother suffering so much! It appears as though she has lymph nodes through out her body including chest (mediastinal lymph nodes). I understand she has had many investigations done but please check if the following have been ruled out: Tuberculosis (through sensitive ELISA test), cancers of lung, esophagus, lymphoma, acute lymphoblastic leukemia, cystic fibrosis, and histoplasmosis. A PET scan may be diagnostic. If this does not yield results then yes, a lymph node biopsy may be required (however, before that it is important to rule out tuberculosis).
Rheumatoid arthritis too can cause swollen lymph nodes and fever. Hence, may be she is having a flare of rheumatoid arthritis. This needs to be ruled out.
Since she is having anemia, the cause has to be found out. Low hemoglobin is seen in low iron, kidney diseases, cancers (leukemias, Hodgkin's lymphoma's, myeloma, and other blood cancers), aplastic anemia and myelodysplastic syndromes (bone marrow disorders), cirrhosis of liver, lead poisoning, and in vitamin deficiency. Low hemoglobin can also be due to increased destruction of blood or its components as seen in spleen disorders, sickle cell anemia, thalssemia etc. It can also be due to increased blood loss as seen in bleeding in your gastrointestinal tract, either due to esophageal varices, polyps, gastric bypass site, hemorrhoids or an ulcer. In females it can be due to very heavy periods. Your mother may need peripheral smears, electrophoresis of hemoglobin, bone marrow biopsy, blood tests for folic acid and vitamin B12, iron etc.
She needs to be investigated in lines of pyrexia of unknown origin (PUO) and hence also tested for typhoid, malaria, hepatitis, autoimmune disorders and infection. Please discuss with your mother’s doctor. Hope she gets well soon! Good Luck and take care!
Is she on any DMARDS for her RA? RA flares can present with persistent fever but I agree with Dr. Have a PET scan done.