Thanks for sharing this. I was an only child. And I had the experience your brother had. It went on for 4 years. Everybody knew what was going on, including my parents. But nobody cared. At one point one of the bullies beat me up breaking my nose. I got blamed for it. I wish I could claim victim status as a result of this experience. But sadly I can't because, over the years, I went on to do even more damage than was done to me. As Leonard Cohen wrote in "Bird on a Wire": I have gored everyone who reached out for me.
I've never told anyone even 1% of all that went on with me. (The bullying was just one part, although probably a significant part.) Even my parents never really knew perhaps in part because they didn't care but also because, over the years, I became expert at secrecy. And now, at this late stage in my life, it's all just too late to bother with & I'm too tired. (Besides nobody else in my life cares still.) I'll take it all with me to the crematorium. I know you wrote that , even as an adult, if your brother is stressed he will suck his thumb in his sleep. I hope that, despite what he endured, life turned out reasonably well for him. My best wishes to you...