so all of this fuss is because of the younger brothers incomplete stories which is which the sister directly made a conclusion that the 2nd prince is an enemy without looking much into it.
also the sister keeps insisting his younger brother to become the duke in the pretense of justifying the...
so the reason why lamia wants to give her position as the duke to her brother is so that the capital won't postpone the investigation on her brothers left hand accident. WTF that's unreasonable, putting down the whole family future just for trivial matters that's not befitting for a future duke.