Secret of the British Royal Family
They're not a real family. They would take the children of the best families from their different townhouses and make their children live together. Some generations, they wouldn't tell those kids they're not family. They let them grow up thinking they're all brothers and sisters.
The last one or two generations, they actually broke this rule. They wanted to be able to raise their own children, so when they found out, they pulled some strings and were able to break the rules.
They have never been a real family before. They usually have to kill one of their own to gain more power. The British use this system because those that kill their own to become king are also more likely to kill anyone around them so their country gets more money. It's all psychology.