Honestly I feel like Cain wouldn't be so stressed nor have to worry about Di dying/ murdered/ becoming a villainess if he had just said fuck you to everyone, took his sister and friend and left.
All these crappy situations are happening because of the "nobility" bullshit. They never received parental love and affection, couldn't make other friends, has to follow strick bullshit rules and manners to fit in because that's what their parents want, have to hide their dreams and ambitions because they don't fit the "nobility" standards, has to stay apart (not by choice) while trying to survive and look after eachother because no one else would.
Cain is so smart that he could probably start a business and survive far far away from these people and the "plot/scary future" coming true with his one and only sister. And also his friend. In fact they'd live a happy fulfilling life.
Easy to say because you are not there. But their world is different from ours, cain's soul came from modern japan, and the story took place in ancient times. So apparently, a kid making business is a NO NO, a newly reincarnated into a new world is a NO NO too, world is too vast for a kid to wander around and bring his sister too.
Their parents are nobility, it's sad but noble's always wants to be IN the society, they want to follow the tred, wanna do what are the known rules, but that doesn't mean they don't love them. They are just too busy to do their jobs and leading their place. And i guess this story is the closest to the one we knew of, sadly. Their kids are the least priority in this timeline, but they love them. They are just like us, they are just new to this life that they are living and there are things they want to learn and Cain is slowly and discreetingly teaching them too.
OH MY GOD!!! FINALLY IT'S STARTING TO SAIL