Does the king see it as just helping the brother out, or does he see this as a form of kneecapping and punishing the father?
Probably both? However I think it was to initially determine the character of the Marquis. Considering the major loss to the city and whilst being saved by the third party (our mc van), from being absolutely slaughtered. The marquis would be too stressed an frustrated to put on a mask and hide his true self infront of the king.
However after him witnessing how he treated his son Murica. Not to mention Muricas & the Marquis's own reaction to being told it was Van that saved them and of Van's accomplishments, probably deduced that whilst the Marquis is a good warrior, his poor treatment of his children and ambitions have harmed his ability to do his duty.
Thus by taking Murica away to Van, Murica can foster their talents better without inevitably being thrown aside or being replaced by a more influenced heir like Murica stated near the end. This also punishes the marquis because he relies heavily on Murica but talks down and doesn't treat him properly like a son or heir. Probably so the king can kinda force the Marquis to see the error of his ways and how he mistreated his family.
TL

R yes, probably.