I am so sorry for this loss in your family. On the 27th of this month, it will be 9 years since I lost my first baby. It was early on, but it was still earth shattering, and it didn't help when exactly 7 months later it happened again. There have been so many nice thoughts from so many nice people on here already.
The best thing that you can do is just be there for your sister-in-law and offer any kind of help or assistance she may need. Early on for me, it was just so hard to hear people say they were so sorry, or just say anything about the baby, but after some time passed, it was nice to know that anybody cared. Don't forget about it, she never will.
Also, personally, I wish people would at least remember me on Mother's Day. They don't have to send a card or anything, but just let me know that they remember me and the little ones that I lost. I think that in the future that would be a nice thing for you to do, just let her know that you are thinking of her on Mother's Day.
I am now a step-mother and that also brings a lot of mixed feelings, but thats another story. I am hoping to try for another baby this year after me and my fiance get married and in some ways I can't wait and in some ways I am terrified because of what happened before. If your SIL decides to try again, that will be another time when she will need all your love and support. Sorry for the length, I just wanted to let you know that I am thinking about and praying for you and your family.