I get it is hard for you, but people who have not been through infertility do not understand as much as they may try and as empathetic as they can try to be.
I have struggled with infertility for over 5 years. For over 5 long years I have tried to have a baby and not had one. I have a 10 year old DN. He was alive before I started trying and I love him to pieces. He is wonderful and we are close.
My IVF failed this week. At the moment I actually cannot bear to see him. The pain is too raw. It is not his fault, is nothing to do with him, but I will cry my eyes out if I see him.
That is with a 10 year old, not a baby. I would not be anywhere near a baby at the moment (not least because I may actually just steal the baby). I do not want to see baby pictures, I do not want to see babies and I do not want to see pregnant women. It is too painful to see it and not be able to have it.
It is not your fault, it is not your child's fault and it is not your sister's fault. You don't have to like it and I can guarantee that she will hate herself for not being able to see you and your child, but being with someone, especially with a baby there and to talk about the baby (there is no way a new mum does not talk about the baby) is just too hard.
I know it is difficult for you and it is hurting you, but you just need to stay patient. She will come back to you, but we are a jealous and bitter breed and it will take her some more time.