I was read from a manga (I was forgot the title) that one of the way to deal with yandere is just fulfill what her/him want at first. Like, win their heart first and after that you can treat them however you like, since yandere actually already fall head over heels at the first place (just their execution is scary, that's yandere for you).
But for this case is extremely risky. I wouldn't talk about defective offspring, moral taboo, or something like that because they are NBR (mentioned at ch 1). The biggest problem is they is still high school student. Well, bro can looking for part time job or something like that since he is high school (most of the part time job place doesn't hiring middle schooler or below). But that's not that simple.
Well, in the end this is just fiction. I haven't read the novel yet. Anyone know where I can read it ?