Firstly, what a great SIL you are to be so concerned with their feelings with everything going on with yourself Secondly, phrases you should NEVER use include:
"Don't worry, your time will come"
"just relax"
"Maybe you need a holiday"
And what I believe to be the worst-"it will happen when the time is right"
They just make you feel worse. Your SIL will always feel uncomfortable when you are around, but If she's anything like me, she will come around. My best friend just had her first baby 5 weeks ago. The trip to the hospital for visits was awful, but you get over it. You have to. We started trying at the same time and when she told me she was PG I completely cut her off. She was so upset, but I couldn't bear to see her. After about 4 months into her pregnancy I gave in and started to be happy for her, knowing that eventually it would be my turn. I have been ttc for nearly 2 years now, but hopefully my luck will turn. FX

So I guess all I can advise is just to give her space if she want's it, and don't be angry at her if she doesn't want anything to do with you at the beginning. And maybe encourage them to talk about it with you, so there's no awkwardness at family gatherings. Good luck with your pregnancy
HTH