I have DS(4) and have had 3 mc since he was born. I've now given up any hope of him having any siblings.
Now, I appreciate I may be very ODD here but I have no idea of when they would have been born, no recollection of dates of conception or anything, there literally is no point what so ever in recording any of that IMO, it would only give me one more thing to be sad about, and I really don't need it.
When my Sis got PG, herself after a loss at the same time as my last MC, I was delighted for her. She wants more, not sure if they are TTC or not.
It wouldn't ever occur to me to feel anything other than happiness for my family or inlaws.
My best friend got pg a month after my last loss, was I sad?, absolutely not.
Did it make me think about what might have been, of course it did, but Would I sit there thinking dark thoughts about my relatives that ARE pg? NO, and certainly never enough to actually take them to task over it.
OK so I've not been lucky, since having DS, and will probably now never be, but why on earth would I have the audacity to in any way impact my sister/sil's pregnancy.
The way I look at it is that there is a reason why the PGs were not viable, that it's for a reason and this is why we never tell before 3m. If it's a significantly late loss, then it's another issue, but we have to face facts that not all eggs will work, not all pgs will go full term.
FWIW, I'm sure she IS highly conscious of your loss and I wouldn't expect her to lord it over you.. if she did, I'd be the first to suggest 'having a strong word' but to go and have a strong word in advance of them doing anything? That's surely not on!
I appreciate you are in a difficult place, but that is no reason for you to impact the joy of others. How would YOU like it, if when you do get PG for someone to come up to YOU and tell you to keep a lid on it?
Sorry, I don't mean to come over too harsh, really I don't, but you have to be philosophical about this and you have to move on. Your SIL has every right to be happy in her 8th month, and hopefully one day soon you will be too.