I know how you feel. People getting pregnant and having babies is a big change and you feel like you've lost part of them even if you're close, because then it all becomes about the baby. I have a cousin with some learning difficulties and although she's not diagnosed with an ASD or anything I have still always felt that she is the only cousin of mine that is the most similar to me (finds certain things hard, not many friends, in simple cleaning jobs, and don't drive), but she often says that she'd like a baby with her boyfriend and I find it hard to imagine it. I think it would take me a long time to accept it if she ever does fall pregnant. It also means everyone will move up one generation (her mother will become a grandmother, her sisters will become aunts, her grandmother will become a great grandmother, etc).
But trust me, you will start to get used to having a baby in the family and you will grow to accept it. You never know, you might find you love the baby and having a baby around gives you an excuse to play and be silly again.

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Female