Right, so NOT brother-in-law, but step-brother or foster-brother.
I think the Jap word is the same, but come on. Are you guys even reading the story you're translating?
Between this and the masturbation thing in the previous chapter, I don't know whether to keep reading. It's not that "the English is so bad you can't understand anything". It's that it's good, but they're making up what's being said, not translating...