Help with traits?

I have created traits so that all of my barbarian factions have tribes that their family members belong to. I also created a trait that that gives bonuses to the chieftain of each tribe.

I have the triggers that assigns characters to a tribe working, but I can't figure out how to make a trigger so that only one member of each tribe can be the chieftain at a time.
Anyone else have a similar problem (or have any idea how to help me)?

You can't unless you link it to the faction leader or faction heir, or use an ancillary to do so.