And it was very easy. I made the sassanids shadow the regular rebels and gave the the worst disloyalty trait to all family members. After a few turns I got the message "A traitor among our people" and a family member deserted with his city and became a rebel.
I only tried it with the sassanids but it should work without problem for all factions. The only problem would be that you can't actually see the loyalty value of a characters for non-roman factions. Anyone know a way to solve this? It might be possible for me to create a trait that is updated every turn and that shows the loyalty value, but perhaps it is unecessarily complicated? We should look for other solutions first.