I am sorry that your sister seems so angry.Your news deserved a more positive reaction, but I think that your compassion towards her situation and the likely feelings she has is really important in making sure your relationship doesnt suffer further strain.
I can kind of identify with her feelings though, and can tell you that in hindsight, I am embarassed to have acted that way towards my own SIL.
For us it was that I wanted to TTC and DH still wasnt ready. His younger brother was in the Army, and when he got home from Iraq, he made an announcement (I" am going to knock her up") which I found offensive. At the same time my SIL had privately told me she wasnt ready to have kids, and wasnt feeling like she would ever want kids and then all the sudden, they get pregnant and call us while we are on vacation to tell us. I cried that whole night. I was angry and jealous that 1) they had beat us to it 2) they didnt seem to take getting into parenthood as seriously as I felt I would 3) my SIL didnt seem to want a baby, when I was dying for one.
It felt like a slap in the face --and I see now that I made it all about me -- their pregnancy news had just brought up a lot of the issues that DH and I felt unresolved about and it hurt that we were so unresolved. After a while, I tried to get more involved and be supportive -- and was lucky that my SIL let me participate a bit in her pregnancy (we went shopping together for maternity clothes) an gave me the benefit of the doubt. I really tried to be happy for them. When the baby was born, I again cried and felt very very jealous but then we went to meet/hold her and I fell in love with my niece.What I learned about myself and my own relationship during that time was that although it was hard to wait, it was the best choice for my own relationship (to postpone kids for a while and work out the commitment and other issues).
Someday, your Sister will probably come to that same conclusion. For now, she's probably just feeling left out and jealous and uncertain of her own future. That's her stuff though -- and she shouldnt put it on you!