1. How is there no complainant? You have yourself stated that a phone call was made to the PS. So the information was provided through the phone call. It is not necessary that there should be a complaint and a complainant. The process of law cannot be circumvented on the ground that there is no complaint made.
2. If the allegation is not supported by proof then the accused will be exonerated by the court. Furthermore, the absence of proof may make it easy for your brother to get the bail.
3. You can go for quashing of the FIR if the allegations have not been supported with cogent proof. There is a good chance that the case may be quashed and the accused are exonerated by the court.