I hope you have found some peace, but you will probably never find the answer. I don't know the previous poster or anything about his background, but can say after 20 years out here, the only thing I've learned is you can't ever know what happened in front of him if you weren.t beside him. But the last thing I would ever do is attempt to post an opinion in something like this. He was a professional and I'm not sure how to say what I'm trying to say.
My son was killed in a single vehicle accident. I practically raised him from a toddler on my cattle truck with me, his booster seat was strapped solid on his side. I've got pictures of him standing chest high to the bottom Peterbilt step waiting for me to pick him up & stand him on each one to get in. He been hundreds of thousands of miles with me & he'd put over 125,000 miles on his explorer in about 20 months. He use to have me give lectures to his friends about driving, they'd want to know how to handle a blow-out, I wanted to tell them how to not overcorrect when they fell off a shoulder of the road. That was my fear & what I felt could save their life, Trey knew this, knew it well, blowouts are easy, jerkin that wheel will get you every time. And from all appearances & evidence it killed him, and yet I know something happened that morning. I have spent time just sitting there looking & the best I can gather a large deer was known by residents to cross from the pasture baxk into the woods about sunup, (it was 540am), he was in his mothers car (wore out,plus he wasn't used to it Mazda vs his explorer), he had inherited my bullhauler right foot & I think had I been in his place (and would give anything to have been) it would have been the same result.
The question is not for your brother, but what happened in front of the other driver to create the situation. Your brother was probably trying to minimize the situation for the driver/traffic behind him-I'd look at the front truck & why did he overdrive his space to cause him to have to lock it down-leaving your blindsided. When in a situation like that(one truck behind another, you have no view of traffic flow in the second truck) you're depending on the pro in front of you to be driving far enough in front of him self to where he would not have to lock it down, thus leaving you nowhere to go. I hope I'm making sense trying to explain what I'm seeing. I wouldn't think for a minute your brother drove into the back of him, I think something happened to make the front truck lock it down & your brother had nowhere to go because he saw no reason to, he probably, nor would I drive in such a way as to make us lock it down when another driver is riding blind behind me-the front truck is like the conductor of a train, if that makes any sense.
It looks to me like a square hit, but that's my thoughts, I don't know if they help,