According to the AA&MDSIF, some studies that followed AA patients for 10-20 years have reported that 35-50% of patients develop MDS or AML. Other studies report only half those numbers.
Although it might seem like a large number of AA patient progress to MDS, it may be that those patients actually started out with hypoplastic MDS. Hypoplastic MDS can resemble aplastic anemia because the bone marrow is essentially empty. In fact, this may have been what happened to me when I was originally diagnosed with AA. One and a half years after ATG and immunosuppressive therapy, I was "re-diagnosed" with MDS. In fact, my disease was probably MDS from the beginning.
At the present time, the only absolute cure for AA or MDS is transplant, but that is not without considerable risks. You have the advantage of having a matched sibling donor available. Another consideration is that even though you might have fallen out of remission, another round of ATG/immunuosuppressives might help you get back into remission. You should take a look at the profile of Marrowforums member,
Andrea Pecor, who has survived AA for many years with multiple rounds of ATG.
Hope this helps.
Ruth