Hmmm based on what you've said, I suspect that you haven't been as sensitive to your sister as you could have been. I'm sure you didn't intentionally hurt her feelings but I think you could have done things slightly differently.
For example, telling her about your pregnancy in person, from what I've heard many women who have suffered pregnancy loss actually prefer to be told over the phone or via a message so that they don't have to put a brave face on and can end the call to have a cry or process the news before replying to the message.
Secondly, announcing your pregnancy on social media - of course you're within your rights to do that, but perhaps the timing of the announcement was unfortunate, or perhaps it just caught her off guard to see it in her feed - she might have preferred a heads up beforehand.
Lastly, if you're contacting her a lot about your pregnancy, it's probably too much and you need to step back a bit.
OTOH, it was kind of you to send her flowers for what would have been her due date, and it was a bit rude of her not to acknowledge it and thank you.
I wish you all the best for the birth. I suggest that you give your sister some space and try not to worry about her too much, you can't expect her to be happy for you sadly, so just be grateful for the support you do have and enjoy baby when they arrive.
Hope your sister manages to get pregnant again and have a healthy baby very soon.