You sound like such a lovely SIL and she will be so grateful for your support. Having had a mc, I would say the following: keep the door open for her, even if she says she doesn't want to talk about things. I often find that when I'm specifically asked how I'm doing, I tend not to want to talk about it and shut it down quickly. Knowing that there is someone there to approach at any time when I feel ready to talk is such a help.
Also, like other people have said, the usual statements can be really upsetting. My Dad's response was 'well at least you know you can get pg' which hit me like a train. It's not just about getting pg, but also sustaining that pregnancy and delivering a healthy baby. I felt completely inept knowing that I had failed to do those 2 things.
The final thing I would say is to try and protect her (where possible) from other pregnant ladies and newborns. This has been one of the hardest things for me. I don't want my DH to visit one of his good friends since they had their baby - the thought of him holding their baby and cooing over it is really hard for me to deal with at the moment. It may seem really irrational and I definitely don't want that baby, but I also don't want him to hold a baby that isn't ours. It probably doesn't help that my due date is 17 days away so it's pretty raw at the moment.
My point is that some of her behaviour may seem unreasonable at times, especially where pregnant ladies/babies are concerned but I've found that I just need to do whatever I can to protect myself. I may lose some friends in the process of that (I dare say they wonder why I'm not delighted when they announce their pregnancy) but I don't have the strength to act any other way at the moment.
Everyone is different so some of this may not be relevant to your SIL but just being there for her and supporting her however you can will be such a big help. Xxx