I think anyone reading Will Clemens' introduction to his 1882 FAMOUS FUNNY
FELLOWS would have been left with the impression that they were somehow
related. Will Clemens included his correspondence with Twain in his intro,
and discusses Twain's geneaology and family in a very familiar tone. He
never makes the claim himself, however. Reviewers of this book would be a
likely source of such a story. Likewise, he does the same thing in his 1892
biography of Twain, using Twain's letters again, and repeating the
discussion of Twain's ancestry and family. Again, he does not make any
claims of kinship himself, but reviewers of that book would be a likely
source of such rumors.

Had there actually been a relationship, no matter how distant, I'd think
Will C would have taken the opportunity to point it out in his 1914 booklet.
If Will Clemens himself ever made such a claim in print, I've not found it.
If you discover he did, I hope you will share it. Good luck,

Kevin Mac Donnell
Austin TX