I'm a sibling of someone with Downs, she's older than me. She's the light in my life. She's guided me all of my life and is probably the person closest to me on earth. I'm so thankful for her. We live together, she's a brilliant Aunty, really close to my kids, especially my teen. She adds only good things to my life, truly.
I'm sure it is a lot to hear and to process if you have not known anything about it before. The older generation were not given the chance to flourish as other children did - there is, broadly speaking, a really big generational difference due to educational opportunities, better professional knowledge & advice and so on. Looking at the older generation as a template for the life of a child now is not a good match, things have changed a lot.
A person with Downs cannot be defined by that extra chromosome. There are, of course, some commonalities but it doesn't provide a type of ability or personality. My sister is really like my grandmother in terms of personality and looks.
The Downs Syndrome Association have a helpline for people in your situation, pretty much every parent there went through your shock so know what it's like.
The sibling part of me feels excited for your daughter. I feel excited for you and send congratulations to you all. I appreciate you may not feel this way. Wishing you all well