Seisou takes place 15 years after the end of the TV series in which Yahiko was ten or so. I figure that by age 25, he and Tsubame would definitely have tied the knot.
That's true. Still though, I wonder if the OVA wasn't hinting that Tsubame was sterile. Surely she and Yahiko should have had a child after eight years of marriage.
In any case, in the manga timeline, Shinya comes along eventually and he and Kenji probably grow up as rivals if not even bitter enemies, as they both desire the Sakabatou and mastery of Hiten Mitsurugi Ryu. I'm sure both their fathers are soooooo proud.
rolleyes Sessha wa rurouni, de gozaro yo.