It has been said that mom alluded to the eldest child causing harm to M. If this were the case, there'd be no need to protect the child, or themselves, from prosecution. There's been times when all of us, cousins, siblings, etc... have played, alone, or together, of varying ages, and could have gotten badly hurt/or died. Parents cannot be everywhere. This goes for me and my own.
I went through a plate glass chasing my younger sibling. I could have easily died, trying to get to him. My brother about knocked me silly, when he nailed me, with a big ole rock upside the head, we didn't think my cousin would survive childhood, and were terrified of his eldest sibling (yet we all played together). None of us died during childhood, but we were injured. Same goes for my kids.
Sometimes elder siblings/cousins/neighbor kids, are too rough with littler ones. My oldest could be rough. I kept a watch but couldn't keep eyes glued constantly. And, sometimes kids do, just fall out of trees, while playing Tarzan, turn their go-karts over, or, get thrown from ponies, etc... and it is what it is. What I'm getting at, is, even if she'd been supposed to watch the eldest, around other children, accidents can still happen. As OP said, he could have been trying out the latest wrestling move, (I've had to come down on mine for that), and hurt her badly, internally, but that would be an accident, and
zero reason to hide her body. There would be
every reason to seek medical attention.
I'm not letting mom off the hook yet. Mothers, (bio parents), can be just as guilty as anyone else re; abusing children in the household and even more-so. I think, if true, and she's tossing some blame on her eldest, she's doing so to save her own hide, and probably that of EK, for being complicit.
Chapter 3, Page 46
Chapter 4, Page 74
Lots of interesting data in this source, from the Children's Bureau, an Office of the Administration for Children and Families.
https://www.acf.hhs.gov/cb/resource/child-maltreatment-2014
Mom blames son for child's death
http://www.ibtimes.com/mariah-woods...-north-carolina-toddlers-death-report-2623894