What if you have a child who is older than your sibling? This isn't uncommon in CK2 at least, so I assume the situation will arise in Imperator as well.
Also, how much legitimacy will that action give you for oratory power and tyranny? It sounds pretty cheap, and I don't want legitimacy to be something like in EU4, where it's just...irrelevant. It should matter. And can the cost of it or gain from it scale to the size of your empire (directly if cost, inversely if gain)? A quick speech might appease a city, but if you control all of Italy and beyond, it should be less effective.
Ninja Edit: you beat me to the punch of saying how much you get, but I'm standing by my belief that the cost or gain should scale in some capacity.
I assume that a successful unhappy heir, with their veteran mercenaries, may eventually come home...hopefully unit experience matters a lot, so that even a single army that went off like that can prove a very dangerous threat to large empires. And they keep the money from their adventuring, right? They have no reason to send it home, after all, except to bribe the populace and especially the armies.
On an unrelated note, I had hoped they would grow on me, but...am I the only one who still doesn't care for the character portraits? The style is just not working for me. It's such a dumb little thing, I know, and thankfully (presumably) mods will be able to fix it, but I just felt like I should mention it.