People are not arrested for murder without evidence. There's no reason you would know what the police have but they certainly have evidence. It may not be the strongest evidence. Their investigation is certainly ongoing while he's in custody. He will get a well qualified court appointed attorney. His attorney will be not be given much of the evidence preindictment but once he is indicted the attorney will get everything and will review it with him. The attorney will most like request an examine trial preindictment to cross examine the police and get an idea of what kind of what kind of evidence they are working with. Expect this to be a slow process.
Macy Jaggers's answer to a legal question on Avvo does not establish an attorney-client relationship. Please consult an attorney who practices criminal law in your jurisdiction for the most accurate legal advice.
All the police need to make an arrest is probable cause to believe that someone has committed a crime. This is a very low standard of proof but I am sure in your case that the arrest is backed up by more than assumptions. I am also curious about bail; if he were charged with murder his bond would likely be much higher.
They obviously had SOME level of evidence/belief your brother was guilty of this crime. All they need to arrest is probable cause. That is a long way from the burden of proof which is beyond a reasonable doubt. I must advise to not talk about the specifics of this case on this, or any other, website/forum. You never know who is reading and if it could come back to hurt you or your brother. Your brother needs an attorney, and he needs one now! If he cannot afford one, the Court will provide him with one. From there, it will be the attorney's job to help get him out of jail and get him found not guilty.