Agreed. I don't remember if the show every said if the company the other brother is the president of is also a passed down family owned business, but if so, perhaps he thought they'd always run it together. But again, even if that was the case just make Rokuhoudou a side/sister company and let Sui run it. In that regards the only thing that comes to mind is that perhaps grandpa wanted Roku to remain separate from the family business and Sui is honoring his wish.

There could also be a jealousy and envy angle. Sui gets to do what he wants, while he has to adhere to family expectations. Maybe he wanted to run grandpa's business to, but can't. So many questions, where's season number 2!?

Well, the last episode had those flashbacks, where Sui kept winning at something and then downplaying it. I think that nothing in particular happened; it's probably more a lived pattern between them. Brother (whose name I can't remember right now) probably got mad at that and feels guilty about it. I suppose the broken cup would stem from that scene (though I have a vague feeling that I'm forgetting a scene we've seen?). It might be that someone would have to take over the family business, but both of them would rather have the establishment, but both feel unworthy of Grandpa, and Sui overcame that first - as usual ("always winning"). There has to have been a scene where Sui tells/asks his brother, and that wouldn't have been easy.

I'm fine with not knowing the details. Narrative was never the main draw of the series for me. I'd be totally up for a second season, though.