Montgomery County police said the Silver Spring teenager, identified as Johnathan Aguiluc, is charged as an adult with first-degree murder and was jailed without bond, The Washington Post reported.
Aguiluc was arrested after paramedics were called to an apartment early Friday and found the infant, identified as Larissa Yanes, unresponsive. The child was taken to a hospital and pronounced dead at 6:54 a.m.
Investigators said the 39-year-old mother of the teenage boy and infant girl, Gloria Yanes, was at work when the alleged fatal beating took place, the Post said.
Yanes arrived home at 5:30 a.m. Friday and at first thought her daughter was sleeping, but found her unresponsive when she tried to wake her and called for help, police said.
Detectives said Aguiluc admitted beating his sister and then covering her mouth and nose with his hands when she wouldn't stop crying.