I struggle with threads like this because I see both sides. I feel for OP and I don't think she's done anything wrong (except to be honest I cringed a little about the flowers on her would have been due date, I really would have hated that when I had my miscarriages, it would have been a punch in the gut tbh).
However, I think in these situations it's really important to be mindful of the effects of grief and sometimes other mental illness which can lead on from that. It took me a lot of counselling and years of depression, sadness, loneliness and yes sometimes selfishness (although it was more self preservation to me), to get over it and I'm not even sure I'm there yet.
People grieving or people who are perhaps depressed following a loss like this are not always rational. Yes she should be happy for OP, yes she should be a supportive sister, yes she should be excited about the prospect of a new niece or nephew but it sounds like right now she's struggling too much to be so and I don't think you can blame a person for that. You can't flick a switch with mental health, sometimes you can't pretend or just slap on a smile.
I did very similar with some of my friends, I withdrew from them, I perhaps wasn't as supportive as I should have been. But at the time I couldn't be, it literally felt impossible. At the time it felt like the only way I would make it through was to ignore the fact that other people were successfully having what I so desperately wanted.
It's not just a case of 'her miscarriage was 9 months ago so why isn't she over it now'. It sounds as though this woman has been TTC since then and hasn't yet. That will also put massive strain and worry on a person.
I just really struggle with these types of threads because I think what a person should be doing and what they are actually able to do are two very different things and I don't think it's anyone's fault, just unfortunate and sad for all parties involved.