The purpose of the conflict of interest rules is prevent the attorney who had once worked for you from using any confidential information he has about you against you in the current proceeding where he now represents the party opposed to you. The typical remedy for that is to get the lawyer barred from representing your opponent. Once a new attorney took over that risk was gone, accomplishing the same thing. The conflict of rules have nothing to do with the conflicted lawyer's legal theories, strategy, etc. And as pg1067 pointed out, the new attorney decides on his/her own what strategy to pursue. It is not the conflict that is your problem once the new attorney took over. It is simply that your brother is suing you, and that would have occurred whatever lawyer he finally settled upon.
Malpractice is a claim that the lawyer was negligent in representing you. So to make that claim you need to show that the lawyer was negligent in the advice he gave you, negligent in his representation of you, etc. You also have to show damages to succeed in a malpractice claim. And to answer your previous question, generally your legal expenses defending yourself are not recognized as damages for which you can sue.