The fact that you are worried about how she feels going through her grieving process shows what a kind, loving sister you are. I was pregnant at the same time my SIL was pregnant with my niece, within 3 weeks of each other. I lost that one, and it really hurt to see her and her pregnant belly after that. About 6 months later I was genuinely happy for her and I couldn't wait to meet my niece. I just needed some time to grieve and to heal, and she never shoved it in my face or talked about it around me, which I appreciate to this day. One of the sweetest things she did - after it had been awhile, she asked me privately how I was doing emotionally. I did not expect that, and burst into tears. That was enough for her - she gave me a huge hug and told me how sorry she was I had lost the baby. One of the worst things is when people just pretend nothing has happened. The giant elephant in the room is unbearably large but people just try and do the polite thing and say nothing, do nothing. Not many people know how alone that makes you feel unless you've experienced it yourself.
There are a couple of great websites around that have a list of what not to say to somebody who has miscarried. I had people say a few things on those lists to me - and they really, really hurt. One of the #1 things people unthinkingly say to a woman who has miscarried: it was meant to be, or it was somehow part of god's plan for me. If it was part of gods plan - explain all the abused and starving children born to uncaring parents. I don't believe god has anything to do with somebody losing their baby.That would just be cruel, and I refuse to believe that god would be capable of such an awful thing. It's like if you had your leg amputated and someone said - it was meant to be! Yup - just part of god's plan! What?? Are you crazy? lol. People usually always mean well, but the best thing to do is put yourself in the couples shoes and try not to use platitudes if you can help it. I hope your sister feels better soon, and you are very sweet to worry about her.
Congratulations on your pregnancy, and try not to feel guilty. Just try to be there for her when the time comes, and tell her how sorry you are. She'll need the support of her sister before too long. *hugs*