Hi C.,
Congratulations on your pregnancy! I think that it is so thoughtful of you to consider your SIL's feelings on this matter, you must be close friends.
We tried for almost a year on our own, plus 6 months of fertility treatments before we got pregnant with our first daughter. I never felt resentful of the people around me (and there were so many) that were announcing their pregnancies, I was genuinely happy for them and just hoping that it would rub off on me.
How did your SIL react when you announced your first pregnancy? That might clue you in to how she will react this time. My best advice would be to tell her about the pregnancy while you are alone with her. This way, she doesn't feel like she has to hide her reaction from the rest of the family. If she is close to you (as you had mentioned), then she will be truly happy for you. You could offer to be there for her with whatever she might need...going to appointments with her or calling her for an update afterwards. I really appreciated when my family close friends took interest in my treatments as they were progressing, it was like having my own cheering section. Tell her that you will be there if she just wants to talk about what is happening to her. Sometimes just explaining the process I was going through to my friends and family helped me to cope because I knew that they cared enough to listen and want to know exactly what each step was as it was happening.
Some people who have fertility issues will isolate themselves from family, especially family members with children. A relative of my SIL's did that and it was pretty ugly at times. I was the opposite and wanted to spend a ton of time with my nieces and nephews that I loved so much. It was so comforting to me. Hopefully, your SIL feels the same way. I actually have a SIL and a cousin that get pregnant the first month that they try every time. They actually felt bad telling me about it, but I reassured them that I was really happy for them.
In your conversation, you might want to mention how you wish that you could be pregnant together, but that you will definitely be there for her when she does get pregnant. Don't apologize for your being pregnant...it is a wonderful thing! Infertility is pretty rough, but I will never forget how important my family's support was (and still is) to me. Just knowing that you care enough to worry about her feelings should be worth the world to her.
Oh, there is a very good magazine about the whole fertility thing called Conceive Magazine. You can buy it at Barnes and Noble and Borders. If she seems open to sharing the process with you, you might want to pick up a copy for her.
Best of luck to you and God Bless you for caring enough to consider your SIL's feelings. All the best to you with baby #2!