Actually there's been quite a lot of discussion about changing the way succession works - it's one of the most common things I hear people ask about. The short of it is that it's entirely out of our hands as modders. There are traits that you'd think control it... but it turns out the traits are only used to reflect the stat boost that comes with the position, and don't actually control which guy is the prince. So it looks like asking CA to change it is the only thing that can possibly work.
As for how the system currently works, I'm sure that's been covered in one of the threads about this in the Citadel, so search that for the answer since I don't remember the nuances of it.
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