So the son seems like an untrustworthy fuck boi and the daughter is kind hearted and well intentioned but has a lethargic disposition and frail body.
Maybe the father should have spent more time properly raising his children and attending to their health? Just a thought. I mean they say "as the head of company he has a lot of responsibilities" and yet ignoring his chief responsibility seems to have created this nightmare scenario for the group/company. Granted we don't know why they're wary of the son taking over, but early development is highly important to one's character development. And based on what's already been said, I find it hard to believe there was ever a healthy family environment (even before the mother passed).