This is a very informative and touching post. I agree with you that usually, people don't see depression as a real illness, but the truth is that it could destroy your life entirely just like a terminal illness could do.
My grandmother was clinically depressed for 5 years when I was a child. I remember her lying all day in her bed, staring at the empty room. She couldn't feel any joy of seeing her grandchildren. Fortunately, she recovered. What helped her was that she had to take care of me for 2 weeks when I was 7. This responsibility magically got her on her feet after five years of suffering.
I hope your brother and your friend get better soon!
RE: Clinical depression in focus [what does science say about brain changes if depression persists for many years]