I am very sorry to hear this. I will keep your family in my prayers. My wife and I had a couple miscarriges before our son was born and we were unable to have another baby after. While none of them were carried to full term, we experienced the grief of the loss much the same.
Your sister needs a lot of love and understanding and will need it for quite some time. If she is anything like Leslie (my wife) she really doesn't need for people to tell her everything is going to be OK (even though with time it will) and she doesn't need anyone to remind her she's young and can still have more children (and she most likely can). The best thing is for you and the family to be ready to give a hug when it's needed, to understand as she goes through the stages of grief, and most of all to let her know how much you love her and that you all are there for her whenever she needs you.
She will probably have good days and bad days for a pretty long time to come. I learned on some days when Leslie seemed happiest was when she was really struggling the most.
I'm not speaking from a position of sage advice. I'm speaking as a man who made a lot of mistakes and learned from them.
We do not know why God allows things like this to happen but we can rest assured it is in His plan.
The fact you posted the request for prayers says a lot about how much you must love your sister. The number of guys who responded with prayers and support shows there are still people in this world who will take time ouf of their days so show compassion to someone they've never met.
You did a very good thing out of love for her. Sometime when the time is right (and you will know if the time is ever right, I can't explain how, but you will know), it may be wise to show her this thread and let her know that she was not alone. The time may never be right, but if it is, you will just know it.
Again, Jason, your family will remain in my prayers. Your sister is not alone, God is with her and there are a number of us praying for her still.