You don't say how long ago this happened. If only recently, I would at least look into whether there is some way of reversing the transfer. Next I would talk with the attorney who made the error and make it their problem to fix. Did the court authorize the distribution of the funds to you? If so, I might ask the Judge's help in fixing this. Bottom line, though, is if you are lawfully entitled to it, you should be able to recover it from your brother's though doing so may take court action and cost some money (nothing close to 1.2 million however!) and/or you may be able to hold the attorney liable and/or obtain his malpractice insurance carrier to do the work to recover the funds for you. All in all, for that kind of money, you may wish to at least consult in detail with your own private attorney to fully evaluate your options and which, in your particular circumstances, may be the best course of action. Good luck.
If I were you, I'd meet with the attorney and ask him how he intends to fix the problem. If you in fact properly reversed an intended renunciation, then you can be pretty sure the attorney would like to solve the problem and make things right.
Good luck to you.
What is not clear to me is the sequence of events. If you signed something that released your share, and then the attorney transferred the money, and after that happened you changed your mind, it wouldn't be an error. If you advised the attorney that you were not releasing your share BEFORE the transfer started, but the attorney went ahead and sent the money to your brothers, then you may have a point. There is also the possibility that your original waiver was irrevocable, so advising the attorney after that may not have been effective to change what you had already set into place. So it is not clear if the attorney made a mistake, or you did.
You will probably need to go to another attorney and lay out the facts and figure out if there is someone other than yourself to blame for this and then figure out what you can do. I suspect that what you signed was a disclaimer and once that was signed it was a done deal and could not be revoked.