I went through the same thing, but my brother didn’t succeed in taking his life. He had a similar profile as described above, and socials played an outsized role in his alienation.
At the time, I quit everything I had on my hands and reorganized my life to be much more present. Living around for a while, trying to engage and be a part of his life as much as I could: sometimes in innocuous forms (“hey, wanna do that thing you love this week?”), sometimes straight up suggesting therapy.
You know what? No matter how hard I tried, it didn’t work. I wasn’t able to connect more with him at the time, nor to change his viewpoints. The simple act of getting in touch with him became extremely hard, to an extent none of my friends, or relatives that weren’t part of the nuclear family, were able to comprehend. Ultimately, I just think he had decided things wouldn’t just stop there, or something inside him held him back among us mortals, and that’s about it.
All I’m trying to say is don’t blame yourself for this.
We’re all doing our best and it sounds like you were already being a great brotherly figure. Blaise Pascal once said: “The heart has its reasons which reasons knows nothing of”: the inner workings of one’s mind (God’s, in Pascal’s case) are too difficult to penetrate for our logical reasoning. We’re just out there on this planet trying to figure out how to help our loved ones, and sometimes it’s not up to us.
You’re not at fault, and I hope you’ll soon find peace.