It would not be a bad idea for your brother to consult with a criminal defense attorney. It is not impossible that he could be charged with some sort of child endangerment charge, if the girl regularly was at his home--i.e. if the circumstances were such that he should have taken precautions to prevent a child from accessing such powerful drugs. If the girl was not normally there, then it is much less likely he'd face liability for that, because people are expected to guard against foreseeable risks, not against ones that should not have anticipated. He is very unlikely in any event to be charged with any form of manslaughter, since he did not himself act--or rather, as long as the authorities believe he did not act or conspire with, assist, etc. his girlfriend to kill her child, he should not face stronger liability. However, because of the chance that he could face child endangerment charges, and the chance that the authorities could, even if erroneously, believe that his girlfriend deliberately killed her daughter and your brother was involved, it would be a good idea to consult with counsel.