Hayden Panettiere Death: Brian Hickerson's Brother Found Her Unresponsive

Brian Hickerson was asleep in another room when Hayden Panettiere went into cardiac arrest ... and sources tell TMZ it was Brian's brother who discovered her.

Sources with direct knowledge tell TMZ ... Brian and Hayden were both inside an Airbnb in Greenville, South Carolina, Sunday afternoon -- when Zach Hickerson entered the residence and found Hayden in a chair in the living room.

We're told at the time Zach arrived, Brian was sleeping in a separate bedroom.

First responders were dispatched to the residence around 1:50 PM and attempted life-saving measures ... but Hayden could not be revived and was pronounced dead at 2:32 PM. Docs state Zach and Brian called 911.

The police report said Brian was inside the Airbnb when Hayden died, though the circumstances surrounding his presence in the home were unclear at the time.

Dispatch audio mentioned a possible overdose, and we know Narcan was found in the apartment.