Have you considered that in a world with eugenics, your brother (either as you knew him, or maybe all-together) would have never existed. With that in mind, I think you have to ask yourself, would you give up the time you had with him? Would your parents give up that time? Would your brother choose not to have lived at all over having lived the life that he had?

Yes. But again, abortion and eugenic murder are orthogonal issues to genetic manipulation. Plenty of good has come out of my brothers disease. My parents started an amazing non profit to serve parents of kids with special needs or terminal illness. I would still trade it all for a cure that changed him and helped cure more.