The facility cannot stop all criminal violence by determined inmates, but is expected to take reasonable steps to prevent or minimize it. The issue then is whether the facility was doing the reasonable things to protect your brother or not. If there were threats against him, if the inmate who attacked him had a record of other attacks or in-jail violence, or if the facility was generally lax about supervisiing inmates and/or weapons checks, they may be liable. On the other hand, if they were doing everything normally or reasonably expected of a jail and despite that, there was an unforeseen attack, they'd probably not be liable.