Is Jaeha the one helping to interrogate him? Sorry im slow

Nope

I want to say he is, because the same hair color, the willingness of Yoseob to talk to him and only rice/food that left on Jaeha's face is not disgusting for Yoseob. But logically saying, most probably that's not him. Because when Yoseob is charged, he is still a high schooler. And Jaeha is just one older than him. I don't think someone that young (at least a high school grad) can be a liutienant. Even if Yoseob has to repeat year, Jaeha's age is still too young for the rank. But anyway let's see maybe there's some other twist that we overlook

Sometimes in order to make a person talk in interrogation they use someone close to the person, someone they feel comfortable admitting what theyve done. And even more so since he cares so much for his brother. All they have to do is make him wear a com and tell him what to ask