RA itself can also cause osteoporosis. I do not know how long it has been known that prednisone and to a lesser extent methotrexate can cause osteoporosis, likely as long as these agent have been in use, 20 or 30 years. However, it has only been recently with the advent of good osteoporosis scanners that we have truly appreciated the extent of the problem. Interest has also increased with the available treatments for osteoporosis and the available alternatives to prednisone treatment for RA.
It is true that since your mother’s osteoporosis is medication induced, you may not be at significant risk. But you should still be screened with a bone (DEXA) scan.