Unfortunately, once charges are accepted by the Prosecutor's Office, a complainant does not have the legal authority to decide to drop the charges. Only the prosecutor in the case has the authority whether or not to drop any criminal charge at that point. However, you can make your desires known to the prosecutor that you no longer wish to testify as a State's witness against your brother. You brother should hire a criminal defense attorney to defend the charges in this case.
Unfortunately, it's not up to you to drop the charges. Only the DA can drop the charges. It will help. If you sign an affidavit of non-prosecution, which is how you let the state know you would like them to dismiss the case. He's going to have to hire a defense attorney. You should work directly with that person to help get the case dismissed.
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Before the case is settled, the prosecutors will contact you to find out your position on the case (and any injuries.) I have been practicing in Harris County for almost 25 years and have found most prosecutors to have the opinion of, "are you kidding me? That could have been me & my brother!" most times (assuming there were no major injuries and no drugs, etc.) Each time i have had such a case, it resulted in a dismissal.
When the prosecutor calls, answer or return the call and talk with them about the situation. It is the only way to help your brother.