Webyoriichi is not related to tanjiro as a forefather, but he is still the ancestor of the latter.
His unborn child and his wife get slaughtered by a demon and his only brother.
Webyoriichi and kokushibo were twin brothers born many centuries ago when the war between muzan and the demon slayer corps was just beginning.
Webkokushibo was born as the older twin to yoriichi, the first demon slayer.
He was born in an era that was called the golden era of demon slayers.