I don't have a brother but when I was a kid several of us in the neighborhood experimented. A lot. I mostly messed around (at age 10 or so, pre-puberty) with my best bud's older bro (about 13 or so) who had hit puberty in a big way. The kid was whacking all day long. He used to j/o and let us watch and/or help. Don't know if the brothers ever messed around by themselves, but I was in experimental sexual situations with them together. Today it would called child abuse, I guess, but I enjoyed the hell out of it.
Can't imagine this is all that unusual. This was back in the 60s when EVERYTHING was taboo. Today, NOTHING sexual is taboo (not at all sure that's a good thing, but there it is).