OMG, OP. To find this thread today is so odd. Today would be the 18th birthday of my full term stillborn grandson.
There is nothing, and I mean nothing more heartwrenching than the breaking heart of a mother in this situation. OP, you recognize how you are feeling, the heartbreak you feel. There is no numerical value that can be given to the degree of your sister's pain in comparison to yours.
For now stay close to your sister. Do nothing but be there when she asks for you. Nothing will console her for a while. She will eventually work through the grief enough to begin to function again.
My son and his wife had another child almost two years later. He was born very premature - 32 weeks, but he survived and is a 16 years old hockey player, as well as his 13 year old sister. They also had a son prior to the stillborn baby.
Funny, that my daughter-in-law had major issues carrying three baby boys. When she became pregnant with her baby girl, it was smooth sailing the entire pregnancy through birth.
I am praying for that little girl for your sister and the mending of her broken heart. Godspeed to her and your family.