Unfortunately there is a good chance that your sister has RA, although it is not 100%. I believe the chance is about 40-60% in most studies. Since identical twins share 100% of their genetic makeup, this suggests that there needs to be other factors beyond genes for RA to develop. There is a consortium of researchers looking at the genetics of RA. I believe that they are looking for DNA samples of twins with RA. The studies are based at the North Shore Hospital in NY. You might ask your rheumatologist to consider entering you into one of those studies.
About Alan Matsumoto, M.D.
Part-time Faculty, Division of Rheumatology
Johns Hopkins University