Such an interesting subject. In terms of birth order/ how close in age people are to a disabled sibling, from talking to other people over the years, including some of my other sisters, I think it can be tougher for younger siblings or those who are closer in age.
When my sister was born I was old enough to understand why she needed more attention and not to find that hard. I think it was more difficult for some of my younger sisters who were very small themselves.
My sister with Downs Syndrome went to the same schools as the rest of us, which was great. My sister was really popular at school and whilst no-one would change that, I think the sister who was a couple of years older felt a bit overlooked in comparison.
I remember talking to someone in another family who was younger than her disabled sibling and she said she found it hard that she got less attention and struggled to understand that.
However saying all that it is so hard to generalise about any of this, and @chissylou is right that we need to be careful not to say this is all about disability, these kind of issues happen in any family.