All you can do is be a good sibling--which it sounds like you are!!!

I just read this book not long ago called "Far from the tree" and it extensively interviews families who have children who are not like themselves, due to disability, genius or gender issues. It reveals a whole range of parental attitudes---from rejecting to ambivalent to heroic. Which covers everything from murder to doing everything possible to support a child who is "different". There is even a chapter on Autism spectrum kids.

Parents have this odd expectation that their offspring will be carbon copies of themselves. Some have too much ego tied up in producing kids.