Just as the Ryu Family and the J Family are sworn enemies, the two top-tier families, the Cui Family and the Fu Family, have also been sworn enemies for hundreds of years.
Other martial arts clans, to avoid involving their families in the warfare between such large families, dare not easily marry into or form alliances with these two great families.
The strength of the two great families is evenly matched, and after hundreds of years, neither can gain a decisive victory, let alone destroy the other, but obliterating these second-tier families isn't a difficult task.
As for hoping that an in-law will intervene, causing a massive family war, that possibility is extremely low...
Rather than being sent to be cannon fodder, it's better to avoid them as much as possible... and that's why no one easily touches this forbidden line.
