Is your sibling planning on having children?
Why don't you want your sibling to have children? Is it because you're afraid they will have a child on the spectrum? If that happens, it wouldn't be your responsibility. It's worse when it's your own children that are on the spectrum or have some other disability.
Or do you feel jealous or bothered by the idea of your own sibling having children? Having children is a big commitment and can bring change, which some people are afraid of when their relatives decide to have children.
Or are you worried your sibling might not cope with having a child once it's born? If you're worried about the welfare of the future child, perhaps you can talk to your sibling and ask them if they will be able to cope. I'm not saying parents on the spectrum are unable to bring up children, but you know your sibling more than I do, so if you feel that your sibling having children is a bit too ambitious as an individual, you could talk to them and remind them that having a baby can really change their life.
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