The great war was a violent time in which clans battled each other for land and power. These clans were normally headed by the oldest or strongest male in the clan, called the patriarch. One of the most devastating weapons in a clans arsenal was called a kekkai genkai, a special ability passed down through clans which activated around puberty. These very rare bloodline abilities had various powers, and the clans with the strongest ones tended to dominate weaker clans. The war came to an end when people of various clans got tired of the bloodshed and decided to pursue peace. One of the ways this was achieved was through marriage, similar to old European royalty.
Kekkai genkai abilities are passed down through the female line. Because of this, women do not marry into other families, as this would potentially give another family access to the bloodline ability. Therefore, a system was developed in which males would marry into families as husbands. Sons, brothers, nephews, and other male relatives marry into other clans and create relationships between competing clans while keep their bloodline abilities within their own family.
However, it is uncertain whether a patriarch can completely trust a male from a foreign family. He could well turn out to be a spy or sleeper agent sending information back to his birth clan. Alternatively, he could also be planning to cause chaos in the family he has married into, by Sabotage or assassination. Because of this, male relatives have no incentive to stay loyal to a new family in which they have few ties to.
How can a patriarch ensure the loyalty of a competing clan under these circumstances?