No family can sufficient control downstream.

The English Royal Family managed it for centuries. The Spanish and French for nearly as long. The Hapsburg Dynasty dominated European politics for longer still. Get into East Asian dynastic rule or the great empires of Ethiopia and Egypt... FFS, the Caesers reined across the Mediterranean for what? Nearly a millennium?

And no, the world is not made up of distinct languages and cultures. All continuums.

There's historical continuums that give birth to discrete divisions through geography and social conflict. The Silk Road connects the length of Asia, but the various mountains and deserts and human fortifications form hard divisions between both linguistics and social practices.

Physical and social division over time create a compact social separation between people in the same way it creates distinct speciation at longer distances and time frames.