Two of my older cousins once intentionally clotheslined me coming through a doorway for laughs, but the only time that I ended-up in bed with one of my cousins was when the older ones were physically-abusing one of the younger ones and he jumped on the other side of me on a double bed for protection. So no, I don't think that a whole lot of that goes on in most families.
From what I have read, and experienced, there is sometimes some sibling to sibling experimentation that goes on right around puberty, I think that my one sister and I played both "spin the bottle", "doctor", even strip poker with the next-door neighbor girl and a kid or two from down the street when we were all 13 or 14, that qualified as juvenile experimentation. But, an ongoing brother to brother forced sexual relationship? No, that is pretty rare and definitely abusive.
Glad that you have found us and that you have found out that your situation with your brother wasn't normal. Hope that you get to enjoy a life free from that kind of incorrect thinking soon. Recovery is possible, with a little hard work and an honest effort, you just have to keep after it. Recovery is a lot easier with supportive people who understand.
"We stay here, we die here. We've got to keep moving". Trucker Mark