Yes, it is possible, the State Attorney has the right to reopen the case for VOP even if there is an acquittal.
Legal disclaimer: This answer is offered for informational purposes only. It is not offered as, and does not constitute, legal advice nor does it constitute an attorney-client relationship. Thank You
Your brother can still be charged with a violation of probation. The reason for this is because the burden of proof is different to prove a violation of probation than it is to prove your brother committed a new crime. When a person is charged with a new criminal offense, the state must prove he is guilty "beyond a reasonable doubt." That means, before a conviction can be attained, it is necessary for the trier of fact (i.e., the judge or the jury) to be very sure that a crime was committed and your brother is the person who committed the crime. On the other hand, to prove your brother violated his probation by committing a new crime, the state's burden is much lower. To prove a violation of probation, the state need only prove by a preponderance of the evidence that a crime was committed and your brother is the one who committed the crime. The trier of fact (the judge in this case) does not need to be nearly as sure as he would if he were deciding whether to convict for a new offense. Accordingly, even if the state chooses to drop the charge, or a jury acquits your brother of the criminal offense, the state could still go forward with the violation of probation and be successful on the exact same evidence.
However, part of your question indicates there was a lack of proof that your brother was driving the car and the implication is that all of his new charges were for driving violations. At the violation of probation hearing, the state will have to proof your brother was driving. If there was, in fact, no evidence that your brother was driving, he stands a good chance to win the violation of probation hearing. Of course, this depends on exactly what kind of evidence the state does have.