I've been infertile for 7 1/2 years. We adopted ds last summer. I can see several things here that are probably making it even worse for your sister. First, she's the oldest. I'm the oldest, and even though I knew it was only in my head, it's hard to shake the idea that you're supposed to do things first and lead the way. To see all of my younger siblings become parents was really hard. There were two times in our journey to parenthood that I had 3 sil's pg all at the same time. If we hadn't gotten ds when we did, I would have gotten to have 4 sil's pg in the same year w/o me! I don't think I would have survived.
There isn't really anything you can do to take her pain away from her. Just validate her feelings. Give her permission to grieve, rage, or whatever she needs to do. In family situations like that, you're under so much pressure (real or imagined) to put on a happy face, to not make the pg ones feel bad. Just be a safe person for her to vent to and say whatever she needs to get out without worry of being judged. Don't walk on eggshells as that will accentuate to her how different she is. But do be sensitive of all the conversations that will revolve around pg. Those are murder for an infertile woman, especially when she's in a group setting. Help everyone be sensitive around things like the baby shower, the birth, possibly the ultrasound/gender announcement if there is one. It seems like pg people want to celebrate (which is surely deserved), but having to throw a big party for every little thing related to the pg can be hard on an infertile sibling. I don't know how your family does it. I'm just thinking of friends having to sit through pregnancy and untrasound announcements in crowded settings where they're barely able to keep their emotions together.
Just let her know you love her, that you wish this wasn't happening to her, and that if you could tak it away, you would. Give her many opportunities to be the favored aunt. Aunt and Uncle-hood is what saved dh and I, and my one bro and sil in particular were wonderful to help their kids develop a special relationship with us.