Just because Kokurou had a painfully tragic past doesn't mean he's excused for becoming a monster and murderer. It makes his rampage believable, but also all the more that he should be put down like the mad beast he has become. His crazed revenge will not bring his sister Nerei back to life.
isn't it supposed to be step-brother instead of brother-in-law? idk i'm dumb
You are correct. Rather than brother-in-law, the correct English translation for Kokurou's relation to queen Bransa should be stepbrother.
This is because in Japanese,
義兄 "gikei" or
義理の兄 "giri no ani" are two terms that could either mean "brother-in-law" or "stepbrother" depending on context. If either term was used in the raws, then brother-in-law is definitely a translation error; especially since we know Kokurou and his sister were adopted by queen Bransa's father, making him her stepbrother and not her brother-in-law.