I have read this thread in floods of tears. Tears I never thought I would shed again. Some awful, terrible comments such as "stealing glory" and "get over herself". Ironically I thought I had "got over myself" many decades ago, until I read this thread.
I firstly want to commend the OP for her sensitivity and understanding. She has spoken so maturely, wiser than her years in many ways.
There are no words to describe the pain and grief and loss surrounding infertility. We tried to have children for well over a decade - sold out house to finance IVF, nearly lost my life twice, gave up my career, lost contact with friends and family. The pain was so overwhelming and my attitude to those who were having children was cruel and evil at times. I have had other sad losses in my life and other close bereavements, but nothing will ever, ever come close to those infertile years. The only thing I can compare it to is having the tragic loss of a close loved one, but having that loss again and again and again, every month that passes by that tragic loss renews and every day that passes it grows in intensity, not lessens a little as other bereavements can do. I could not be around children or babies, and some were worse than others. Not for the child themselves but as others describe maybe the attitude of a MIL, or other adults. It was unbearable and I was very, very much more selfish or unreasonable that the OP's SIL is. I was unreasonable to my husband also. And as I say that grew and grew as very month, every year, every decade passed. And the guilt, not only of not being able to give my husband a child, our grandparents grandchildren, was immense. But more than that. It was as if all the generations that had gone before, throughout thousands of years, was coming to an end because of me. I expect that sounds very dramatic. Yes, it does. But it was the truth of what it was like. Every day.
Some people have suggested that the OP brings her lovely baby to the party anyway. Others have suggested the baby and her Dad wait in the car, though it is then likely that others would innocently suggest the baby is bought in. If I were the SIL and that happened to me, I think I would died, I would have wanted to. As I sit here all this time on and remember what it was like and I cannot imagine a worse situation that that. A baby I had not anticipated coming into the safe haven of my home. All these years on I can feel an echo of that horror., though am glad for the understanding of the OP and not wanting to do that.
I wish you well, OP, and your baby and husband, and hope that your brother's birthday celebrations go well for everyone concerned.