I nursed two babies, and especially with my first, I wasn't prepared for how much it was going to hurt.
First of all, I'm small, and all the sudden, I had these huge boobs. I didn't own a single thing that would fit around them and my back was killing me. Then, the nipple thing. I cried and dreaded feeding my baby. I seriously just didn't think I could take it anymore. It was right about that time that things started getting better.
My second baby didn't come along until about 10 years later, but I was prepared, emotionally, for the initial pain that would come. It did, and I survived it. I breastfed my son for 15 months.
I would suggest that your sister call the hospital where her baby was born.
I worked in the OB ward at my local hospital and the nurses are so great. They also have resources for lactation specialists who will even do home visits to work with you and get you through those rough first couple of weeks. I dealt with nervous, frustrated, and tired mommies all the time and the nurses followed up with them right away. They really do care.
Have your sister ask for help. I'm positive there are people who will be happy to support her through this. It could be a latch issue, it could just be soreness. It's been my experience that the soreness does go away, until the baby starts getting teeth, but that's a separate topic. :)
Congratulations on your new nephew.