Where did you see 夜雀 is a word for younger step-brother?
Anyway, despite the fact that some specific words come up when you search for "step-brother", etc., Japanese people do not really get specific with this relationship as much as westerners do it seems. I asked some Japanese, and it was difficult to get them to understand what you wanted to do and why their suggestions felt inadequate.
If you're in a beginner's Japanese class or something (you didn't really say) I would think that a descriptive phrase would be more effective, because anything you use that is exactly specific is going to be obscure to Japanese people anyway.
So something like 違う両親の弟, etc.
Though, I mean, since you said this is a class, can you just ask your teacher?
I did ask haha, she didn't exactly have an answer, so I figured I would ask here. Thank you for clarifications :)
Ah, I see.
Other Japanese people I talked to said stuff like 義兄弟 is fine, and you can specify 年下 or 年上 or something. They said it doesn't matter too much to Japanese people that 義兄弟 includes people who aren't stepbrothers. It's all the same category in Japanese, I guess.
Japanese Presentation on Family
I have to do a presentation in my Japanese class on my family members. I know there are words for younger brother and sister as well as younger step-sister that would work, but "夜雀" for younger step brother is coming up as an obscure word? Is there another way to say this that I just can't find or would it be better to just use the obscure version?