It's good of you to be sensitive, and every person is different, but your sister may come around. I have a good friend with a developmentally disabled child. I was always careful about discussing my HG+ child around her before we became good friends because I didn't want to make her feel bad or sad. But because we were homeschooling, she asked me a lot of questions and I told the truth in as unassuming a way as I could.
She was one of the people most understanding about DS8's needs when I talked with her about them. She is able to accept that not everyone fits neatly under the big bump in the Bell Curve.
I always try to be sensitive to her feelings, of course, and I am not a competitive person when it comes to kids anyway. I hate that stuff. And she is very much a person who accepts difference as a matter of course, so that's part of it, too.
Her child is older, so maybe with time it will get easier with your sister, too?
I'll be hopeful for you.
