I think a second opinion is the smart and safe way to proceed. Depending on the location and the size of the mass there may be no safe way to resect the tumor but finding a good neurosurgeon and arranging to send him/her the MRI would provide you peace of mind that two specialists concur on the diagnosis.
My daughter is 34 years old and has had two surgeries to remove a 5.5cm mass from the left frontal lobe region. She had the initial chemo and radiation regimen and is about to start her 3rd monthly chemo treatment at 3x the dose she had during the post surgical treatments. This will last for 12-18 months. I hope your sister agrees to that second opinion soon but she will ultimately need to make that decision and everyone will need to accept her wishes and move forward. Best of luck!