L.F.
You have already gotten some great advice. I am so sorry that this happened to your sister.
I've got news for you. Your life and all of your future plans have been permanently derailed. There is no way you can move forward without having the loss of your sister's baby affect you.
My sister gave birth to a beautiful full stillborn girl just a few weeks after I found out I was pregnant with my second child. I was able to fly out in time to see her and to hold her daughter. It was the most unimaginable sadness I have ever felt, and I have been through a lot!
No cause of death was determined for my niece. She looked perfect in every way. She just quietly slipped away a few days before her due date. For the rest of my pregnancy, I had to lie awake wondering if my baby was going to die. And every morning, I woke up and realized that my sister also wakes up every morning and realizes that her loss wasn't just a bad dream.
I called my sister everyday for the next few months just to check in and let her talk. I helped her plan a memorial service for her daughter. I sent books and bought her clothes. I honestly didn't know what else to do for her. Eventually, after my sister went back to work, life resumed for the most part. When my son was born, it was very difficult for her to be happy for me. When I called her after giving birth, she could barely say anything to me. I didn't take any offense to it. I knew it must have been like a punch in the gut.
Anyway, you have to do what you can to take care of yourself and your baby. And try to be there as much as you can for your sister. As far as your wedding and baby shower are concerned, they will be just a little bit more somber because of this loss. Other women who have experienced this kind of loss talk about "the new normal". Your sister will never be the same person as before. You may never be the same either. My deepest condolences.