I was interested in rereading this because I was a lot younger the first time I read this, but I remember loving it enough that I went through it multiple times. I do see the appeal, still—it has childhood shenanigans, good sibling dynamics, and a main character that's easy to root for.  But it is very much a product of its time. I can't comment knowledgeably on the ableism or lack thereof, but there are reviews that dig into this aspect specifically, and I highly recommend you look into it before deciding if you want to read this book or not.