Well done for coming in here and sharing the story of you and your sister, i cant imagine what your sister must be going through, and you too, you are carring around so much guilt because of your beautiful healthy baby and should be able to enjoy your baby.

I dont know whether this would help, but from what you have written the only time that she acknowledges that there is a problem is when she is out drinking, not that i recommend going and getting her drunk, but is there anyway that you can have an evening with her and have a couple of drinks and chat to her then?
She has built up so many walls around her that she is a fortress and a bit of alcoho may make her relax those walls
Honestly i cant believe that i am recommending that, but i know if that was my sister and she had all those walls up and pretended that everything was OK, the only time that i would get a glimmer of what was wrong would be if we had a couple of drinks (and it would be late in the night).

The thing is you need to get her to talk about it.
Maybe she is being so strong (or trying to be) is because of you... she doesnt want to hate you or your baby, but deep down there has to be a bit or resentment, not directed to you, but for what she lost and you have this happy and healthy baby iykwim.

As Jess said, sometimes it is easier to open up to others that dont really know you...
There are women on here that have had similar experiences, they can understand where she is at... i couldnt even pretend to understand the grief that she must be harbouring in there somewhere, maybe sharing her story with others and or hearing similar stories will help her realsie tat she is not alone and we are all here to help....

I cant really offer anything else.