Hi Gina,
It sounds like your sister has either one of two options: a plugged duct which can develop into mastitis or a yeast infection of the breast, called thrush.
1) Plugged duct - the best remedy for the plugged duct is to apply heat (hot shower works well) and massage the breast and either manually express or nurse the baby. It is important for her to use heat and not cold as this will allow for easier passage of the plug. She should also try nursing from different positions, i.e. football hold, cradle hold, lying down. This will ensure that all the ducts within the breast are emptied properly. With a plugged duct the more she nurses the better and always w/ warmth and massage. If she has developed mastitis she would exhibit a fever, body aches, and a red, hot and sore breast. If she doesn't feel better within a couple of days I would advise her to see her doctor and a lactation consultant to work on the latching. The latch should be unpleasant for the first few days, but it should definitely not hurt. For mastitis, her Dr. will prescribe an antibiotic. Mastitis does not affect the milk, nor can she pass it on. The best thing is to nurse more often to make sure it doesn't reappear.
2) Thrush or a yeast infection of the breast is very painful. This occurs when yeast from the baby's mouth enters the breast through small cuts in the nipple. If the baby has a poor latch, and the nipple has been damaged, it is very possible then that she would have this. If it is in the beginning stages, she would only have pain in the nipple area, however, if the yeast has traveled into the breast tissue, then she would feel the pain deep within the breast. The pain usually occurs at the start or end of the nursing session and is quite painful. If this sounds more like what she has, she definitely needs to see her OB, so they can prescribe either Diflucan, or a topical cream called triple nipple which they make at people's pharmacy. Some people have also had success w/ gentian violet, however, it stains clothing and the mouth of the baby. With thrush, the baby may or may not have the infection to have passed it to her. If the baby has it in his mouth, she would see white, creamy patches in his mouth, and/or a prolonged diaper rash that doesn't go away w/ cream. Yeast is very, very, persistent so if she has it she will need to continue treatment to avoid her passing it to her son, and then him re-passing it to her.
Either way I would suggest she visit her doctor, and a lactation consultant to make sure that the latch issue is not contributing to any of these issues.
Best of luck, and let her know that it does get easier with time, and although both of these are painful, once she gets past them, she should enjoy nursing..Like I said if there is pain, there is definitely something wrong. Hope this helps!