"Gideon Prewett, it took five Death Eaters to kill him and his brother Fabian, they fought like heroes..."
-- Mad-Eye Moody (OP9)
The brothers appeared in a photograph of the Order of the Phoenix members that Mad-Eye Moody showed Harry at Grimmauld Place (OP9). Soon afterward, Molly Weasley had trouble getting rid of a boggart that appeared as a series of family members lying dead, perhaps because she was traumatized by the deaths of her brothers years before.
Hagrid mentioned their deaths when he first told Harry about Voldemort: "No one ever lived after he decided ter kill 'em, no one except you, an' he'd killed some o' the best witches an' wizards of the age -- the McKinnons, the Bones, the Prewetts -- an' you was only a baby, an' you lived" (PS5).
After Antonin Dolohov escaped from Azkaban, his picture appeared in the Daily Prophet with this caption: "convicted of the brutal murders of Gideon and Fabian Prewett" (OP25)