I don't wish to sound negative. But I can't see this idea working. It would make the game highly frustating. And the chances are the AI simply wouldn't know how to cope with it. I can think of so many problems and drawbacks, and I see little gain. Already there is a drawback for not having a family general, so the incentive for using a family member as a general is already there. All this would do is slow reinforcements, and force you to micromanage all the reinforcements with a spare general. And many people think there's more than enough micromanagement in the game as it is.

And what if a family member dies of old age in the middle of an enemy province? You have a helpless stack of men that you can't move or even disband. And the idea of removing assassins simply to make this idea feasable is madness. So basically you get a ton of huge gameplay flaws introduced to "fix" something that isn't even broken. Right now any decent army will be lead by a family member anyway, unless it's the AI. And fixing the AI is a game patch issue, not something to "fix" by changing a gameplay element so radically