I'm not sure how this could be implemented. Are you asking that a random chance could allow you to use a factional as a general? Or that family members can all do that?

Consider how many monarchs didn't lead their own armies into battle. Quite a bit. I don't think that faction leaders leading their men would be a very fulfilling idea anyways, since RPing has been toned in ETW as well. Your monarch simply is a character portrait, not THE most important man in your kingdom.