I had a similar situation last August. We were trying to get pregnant, I had a positive test, and two days later I started spotting and passed some clots. I took a test and it was negative. Because of the clots, my doctor told me to go to the ER. It turns out that I did indeed miscarry, but because it was so early on in the pregnancy there were no complications. We were able to try again the next month and got pregnant right away, and now we have a beautiful 4 month old boy. That does not mean that the miscarriage was not hard or that I did not grieve, but with time and distance it has gotten easier, and of course now I have my hands full with my beautiful son! That was actually the second miscarriage I'd had (the first one was very early in our marriage when we were NOT trying to conceive), so just because you have one (or even 2) miscarriages, it does not mean you won't be able to get pregnant. If that is what has happened for your SIL, just be there for her and support her in her grief. It happened so early in my pregnancy that we had not even told anyone we were pregnant yet...I only called my mom from the ER to tell her what was going on. I wish more people knew so that I could have had more support at the time instead of having to grieve privately.
When someone miscarries so early, it usually means there was something that wasn't right with the embryo and it would have had a lot of complications. Sometimes it is nature's way of keeping us from even greater heartache if the pregnancy had gone any further. You can google "early miscarriage" and find out more.
I will be praying for your SIL and that everything will work out for the best!