My brother died when I was 12. He was 8, of cancer (different type than OP's daughter but cancer all the same). My relationship with my mother afterwards was never quite the same, though we were never estranged.
My mother poured a lot of herself into my brother's care, my siblings and I often felt like afterthoughts, she didn't go to concerts and games for us because 'someone' had to take care of Ricky when he wasn't up to going. As an adult it sounds reasonable enough. So Dad was the one that brought us and watched.
After Ricky died, honestly yes it seems like the parents got most of the support, kids were expected to just kinda deal with it. "Helpful" relatives would say at least you get to grow up. Mom lost herself in her grief, it seemed that she had wrapped so much up in Ricky's survival that when he didn't, she didn't know what to do except fall into a hole. We'd try to get her attention, we were her kids too. Dad wasn't the most patient after Ricky, he lost a son too, but his attention was easier to get. It may be because Dad was more task oriented, get A, B, C done. Get the kids up, to school, to practice, teeth brushed, off to bed.
There's also nothing much worse when you're a child and you hear your mother cry and cry, and you can't do anything about it because all you want to do is cry and cry too, but your mom doesn't really want to look at you (in a child's view, remember) because you're here and Ricky's not. Sometimes those words might escape out of her mouth and into your ears. She didn't mean them, but it's still a blow to a child.
And maybe Mrs OP had to "go away" for a week or longer to "get better" because of the grief resulting in PTSD, but a child sees mom going away because she can't deal with the kids that lived, so it's our fault we weren't good enough for Mom then.
As an adult, you can start to filter through some of these things, but even in the best therapeutic models, that little voice is still in the back of your head. And sometimes, even with all the work, you swear your mother looks at you like that too.
So that's the view of a child that survived a sibling. Hope it helps.