A Bronx baby sharing a bed with her 5-year-old cousin was found dead early Monday after the older child accidentally rolled over and apparently smothered her, police sources said.

Six-month-old Ziebella Thorne-Young was found unconscious at 1:15 a.m. inside an apartment at Butler Houses on Webster Avenue near East 170 Street in Claremont Village, police said.

The sleeping tot was sharing a small bed with her 5-year-old cousin when the older girl accidentally rolled over onto her as they were sleeping, according to police sources.

First responders rushed the infant to BronxCare Health System, where she was pronounced dead, police said.

The medical examiner will determine the cause of death, but cops have not ruled the incident suspicious, and sources believe the cousin accidentally smothered her.

The young girl was staying at her aunt’s apartment when she was killed, sources said.

Her mother had previously been reported to ACS, but the details of the complaint and whether its related to the incident were unclear.