[FS]

Well, like I said, Gaiman thanks a Bernie Wrightson for Cain & Abel in the credits page of the first Sandman volume. I couldn't find that name in House of Mystery # 2 or 3, and #1 has wandered off somewhere.

What I figure happened is Gaiman simply asked Wrightson if he could use his character. In House of Mystery, Cain is just a storyteller, so there's not much background to go on. Gaiman probably just took that character, gave him his brother Abel (to equate him to the Bible story), and moved him from the House of Mystery - to the House of Secrets, in Sandman. He's still kind of a keeper or stories there.