They established sibling is super sick/theoretically dying earlier though?

That's why he wants to 'make MK tough to protect the family' for when he dies. Which is stupid as fuck what he does, not least of which potentially killing MK with many of these duels that are suppose to magically make MC strong but could have easily killed MC if MC hadn't already been strong.

Along with certainly the strains of near mortal combat surely puts em at further risk hurrying along death. Sick people should not be fighting all out and taking injury.

Also sister doesn't get this alleged "training" which defeats a large part of the point of having the family be safe after his death.