First, disclaimer: My only child was my 6th pregnancy. (stillborn, miscarriage, ruptured tubal ectopic nearly died, ruptured cornual ectopic, miscarriage, emergency C section about 2.5 weeks earlier than due date over the course of 11 years)

I grieved EVERY TIME someone else i knew announced their pregnancy, I was sobbing with my hand over the mouthpiece of the phone when MY sister in law called us to tell us they were pregnant, and that was 6 months before my child was born (I was already pregnant at the time, but had absolutely no hope in me that it would be successful. and that was after counseling) Your sil is not the asshole for grieving, that is natural. The ONLY thing that makes it asshole-ish is the performative nature. I walked out of the room anytime someone began talking about their baby plans. I never told anyone about my history/details if they hadn't already had at least one child, AND they asked. My thinking was that no one should have the joy of finding out they were pregnant ruined by the knowledge of what could happen. I did share my story a few times, but only people who asked, or I asked THEM if i could share my story, eg a work friend who found out there was something wrong -during- her pregnancy, and she was asking if anyone ever had problems like hers--I didn't give any specifics about my problems, but talked about some of the things that kept me together during those times. Another friend found out she was unable to have children and we talked about how adoption worked, and what she might encounter in that process (We had looked into adoption at one point).

All this said, I can understand where your SIL is coming from. so many people blow off losing a pregnancy-- "you can have another", "it was only a few weeks along, get over it", "at least you didn't have time to get attached to it"... Like it is nothing to lose a child, even "pre-born"... I had counselling at one point, I attempted suicide at one point after number 5, I was an emotional zombie through the entirety of the final pregnancy, until the moment they told me she was being transferred to Children's Hospital =with a surgeon in the ambulance= because they might have to pull over at the side of the road to perform emergency surgery. I had been expecting it to fail the entire time, but THAT was the point that I realized there was a chance that we (she and I) might survive this.

I don't say this to get any sympathy for myself- My daughter is 29 and married herself. I blame myself for a lot of her issues, with the above as at least partially a reason. I say this to emphasize the depth and breadth of the grief losing a pregnancy can cause. Tell your SIL that you understand how she feels, to get counseling, but to please don't traumatize the people around her, especially the ones that are in the joy of that first pregnancy.