ssdavi,
Many patients respond quickly to ATG if they are going to respond at all, but there are exceptions. Some patients are "slow responders", which is why many doctors use the 6-month guideline. After that point, if there is still no response, they may recommend a transplant or another round of ATG. (They may also recommend more "wait and watch" for patients with "livable" counts, but probably not for transfusion dependent patients.)
It's great news that your sister is a match, but there are still good reasons to avoid the risks of a transplant if drug treatments might stabilize your counts. At age 51 you are not in the "senior" group, for whom full transplants are often ruled out, but neither are you in the child/adolescent group, for whom transplants have the highest success rates.
You are fortunate to have a few choices, but that makes it harder to decide what to do and when. For example, if your ATG response finally kicks in, or if another round of ATG could restore your blood counts, you could avoid a transplant. But if you will eventually go to transplant, it's statistically better to have had fewer transfusions. It almost seems like you need a crystal ball to know what to do, but that's where the experience of the doctor comes in. A good doctor who has helped many aplastic anemia patients will have learned how to balance these tradeoffs.
When you have doubts about your doctor's advice that's a good reason to get a second opinion, especially if you aren't already being treated at a major center or by a doctor with aplastic anemia experience.
I suggest that you read the
Response to ATG thread for some other perspectives on ATG responses.