The two brothers are DC Comics' Cain and Abel, based on the Biblical characters of the same names. From the 1950s through the 1980s, they were the respective hosts of DC's horror anthologies House of Mystery and House of Secrets.
The Sandman reinvents them as more mature, developed characters who have been transported to The Dreaming as storytellers, where Cain seems compelled to continually murder his brother in a reenactment of the Biblical story. Cain's pet gargoyle, Gregory, is transformed into a much larger beast.
Read the original story in Genesis chapter four at Bible.com.