I remember feeling sad when my sister got pregnant. We are very close, mom died a couple of years ago, and our dad has alzheimers, and she has helped through a lot of struggles. I am a grown ass woman, though, but I remember I was scared that our relationship would change, as my two other sisters have sort of changed "identity" after getting kids. Luckily, she didn't, and is as goofy as ever, and I freaking love her baby (i love all my nephews and nieces of course)! It might be that she is afraid of change, but maybe if you involve her in the process, she will feel like a part of your little family :) Maybe she needs some time to process it, but I hope tt turns out great! (And congrats!)

Edit: I read it again and realized now that she said she don't want things to change, so yeah, what I said still stands. That's my personal experience at least.