I'm so sorry for your sister and her family, including you. I had a miscarriage in early January, at 10 weeks. In my case, it was what used to be called a "blighted ovum" and is now usually just called a "chemical pregnancy." Basically, I had a bum egg--it was fertilized and implanted, so my body created the placenta, which created the hormones to make me test positive and feel pregnant. However, when I started spotting badly and went for an ultrasound, there was no fetus, not even a yolk sac. In that case, there was only the placenta to lose, and I elected to go naturally. I also elected to go out of town less than a week after the diagnosis, with my doctor's approval. However, as I said, I didn't have a body to try to pass, just the placenta. It was still gross! However, we made it through, and now we're trying again. Your sister needs to talk carefully to her physician, but my guess is that if she waits for her body to miscarry naturally, she shouldn't leave town until the biggest part of it is done. She'll have some bleeding for a long time, and that's okay for travel, but she'll want to make sure the basic miscarriage is over.
If she can't leave right away, I encourage her to get away as soon as possible, and go to her "happy place." I was lucky enough to spend a week on the beach, and I just had to deal with a heavy period while I was there. The worst of it was over. Being on vacation really helped us through the grieving process, I believe. I wish her good luck, and all the best.