Pretty much what the description of the story said but this isn't an actual story and instead a prompt one if you want to view it as that!
But the basics of the story is that SQH dies and reincarnates as some unknown character named Xīn Donghai who's the kid of Lǐnguāng-jūn. The story would (if you want) start off with him learning pretty much how to not die but either way he's a few years older than MBJ and of course you could always get ride of the fact that they're siblings but at some point in the story MBJ realizes that SQh was (forcefully) helping Lǐnguāng-jūn and 'kills' SQH (everyone thinks SQH is dead but he survived the injuries somehow) and this is when he'd go to the human realm and become a cultivator. The rest is basically up to you but at some point he does reunite with MBJ and messily clears up the misunderstanding.
But this is SQH there's going to be a lot of misunderstandings and issues he would accidentally cause