My brother is depressed and I don't know how to help

Growing up, there was always a little inkling that my little brother was different from myself and my other brothers and sisters. The situation around him growing up, his relationship with our parents, and how he was in school was always a bit off. Now that he's in sixth grade, he's been in a ton of incidents where he's either fighting, skipping class, or just not taking his academics seriously. After multiple visits to the psychiatrist, he was diagnosed with depression and bipolar disorder. After hearing the diagnosis, I thought about times I didn't react to his outbursts the best. He would intentionally irritate me to get a reaction, and antagonize me to the point I couldn't take it anymore. We rarely fought, but the things I said and the few times we threw hands, I look back at now and wonder how much control he really had over what he did and if I was in the wrong. I can't take back all those moments, but even now I don't really know what to do going forward. We're civil, we make jokes, play sports and video-games, but he doesn't talk to me about these deeper things that he's going through. I can't expect things to change overnight, and it's not like I talk to my older brother and sisters about these things too, but I want him to feel comfortable coming to me when he feels sad.