First time I heard the Dead, but it was a friend's album. First album I bought was "Skull & Roses." I was hooked once I heard that one.
One thing I always say about the Beatles is that they were not a rock & roll band, they were a cultural phenomenon, and "rock and roll band" was a subset of all the other things they were.
The "Psychedelic Royalty of San Francisco" was essentially Jefferson Airplane, Quicksilver Messenger Service, and the Grateful Dead.
Jefferson Airplane mastered the art of 2-to-3 minute hit singles. Quicksilver...viewing them on the basis of the first two albums...wasn't built to last, especially after Dino Valenti got out of jail and rejoined the band.
But the Dead...what a tenure. When they were "ON," no one could touch them. And the whole "figuring it out on stage" thing...Weir learning to play slide guitar onstage before he ever considered learning it BEFORE going onstage...there weren't a lot of bands that had that kind of fearlessness. And they spawned SO MANY disciples, "jam bands" like Widespread Panic, MOE, Phish...
My condolences regarding your brother. If you saw 300 shows, no one needs to tell you what this band was all about.