My sister has stage 4 cervical cancer. She has a secondary tumor, 5cm, on her adrenal gland. We know it is terminal, but can we expect months? Years?

It depends. Her treating doctor can answer this question better. Not all stage-4 Cervix cancers are fatal. So the answer to your question depends on her exact stage(localized versus metastatic) and also on her cancer's response to treatment. Sometimes adrenal tumors can be removed surgically provided the pelvic tumor is under control.There are also new treatments coming along for cervical Cancer(e.g. Avastin)