Authorities arrested the 12-year-old brother of an 8-year-old girl who was mysteriously stabbed at her home in a rural Northern California community last month.The boy, arrested on Saturday, was not identified. He will be charged with homicide, Calaveras County Sheriff Gary Kuntz told reporters Saturday night.The April 27 attack on Leila Fowler set off a massive manhunt in the tightknit Valley Springs community of about 7,400.