I'm sorry to hear about your brother's situation. Unfortunately, if he can't make bail and has been charged with a crime, they can hold him until his case is resolved (and longer, if he receives a sentence of incarceration) or until his bail status changes. Typically, he will be assigned a public defender if he is found to be indigent while incarcerated. If one is out on bail, they will have to actually apply for a public defender, but if someone is in jail, one is either assigned while he is in jail or by the court at his first appearance. There is not really much that a person other than the defendant can do to secure the public defender's representation, but you can always call to find out if there is any financial paperwork or anything that they need to move things along (persons usually have to prove they can't afford an attorney to get the PD):
Once he has secured representation, his attorney can try to file what is called a bail review. Essentially it asks the court to lower the bail amount that was set originally. If your brother can't afford any bail at all, the attorney can ask that he be released on his own recognizance (meaning without money, just on the promise that he will show up for his court dates). The likelihood of a bail reduction being granted depends on a few factors...namely, the danger to the community if he is released (i.e. whether his charges are serious and what his prior record looks like) and what the likelihood is that he will show up to court when he is supposed to. If the bail review doesn't work, the attorney can either try to expedite a trial or, if your brother is interested in a plea, to work out a plea deal where he gets probation instead of jail. Such a plea deal would also allow him to get out of jail even without the bail review. But again, the likelihood of him getting a more lenient sentence depends on the severity of the charges and his record.
Unfortunately, a bail review or a non-jail plea deal at this point is the only way to get him out of jail if no one can post bail on his behalf. Otherwise they have the ability to hold him indefinitely until his case is over. I'm sorry I can't give you better news than that.