Congratulations to your sister on the new baby! I hope she can get the hang of breastfeeding soon. Here's some information from a book of mine, that I think your sister might find helpful.
"Sore nipples
'Breastfeeding is something I always wante dto do. But my nipples have become so excrutiatingly sore, that I'm not sure if I can continue nursing my son.
At first you will wonder if your newborn will ever catch on; then before you know it, he's suckling so vigorously your nipples become sore, even painful. The problem is not uncommon, and in fact most women suffer from at least a little nipple soreness before breastfeeding becomes well established. But in the vast majority of these women, soreness peaks at about the twentieth feeding, nipples start to become less tender, and nursing becomes more comfortable. For some women, the nipples get worse before they get better, cracking, becoming equistively tender, and sometimes bleeding. Nursing sessions can become very difficult.
Fortunately, there is relief at the end of the tunnel for these women, though sometimes not for a month or six weeks. In the meantime, these precautions will help alleviate the discomfort.
Be certain your baby has most of the areola (the dark area around the nipple) in his mouth while nusing, and not just the nipple. He will need more breast below the nipple in his mouth than above, so you will always see the areola where his nose is. His chin should be tucked into the breast. Not only will his sucking on the nipple alone make you sore, but he won't get much milk. If engorgement makes it difficult for him to grasp the full areola, express a little milk manually or with a breast pump before nursing to reduce the engorgement and make it easier for him to get a good grip.
Expose nipples to air whenever possible, but especialy after nursing. Just lower the blinds and leave the flaps of your nursing bra down as you go about your business at home.
Remove any waterproof lining from your bra (again, to encourage air circulation) and don't insert waterproof pads
If only one nipple is sore, don't favour it by nursing more on the other one. But do first on the less sore one, since your baby's sucking will be most vigorous at the beginning of a feeding session. If both nipples are sore, alternate the breast you start with at each feeding.
Let nature - not cosmetic companies - take care of your nipples. Nipples are naturally protected and lubricated by skin oils. Rubbing hind milk (the milk that comes at the end stage of each breastfeeding) over your nipples is recommended. Clean the nipples only with water when you bathe - never with soap, alcohl, tincture of benzoin, or premoistened towelettes. Your baby is protected from your germs by antibodies in your milk, and the milk itself is sterile.
Rotate nursing positions so a different part of the nipple will be compressed at each feeding.
Relax for fifteen minutes of so before feedings - listen to music, watch TV, catnap, do relaxation exercises, have a hot drink, hand express to allow the milk to flow and moisten the nipple - to banish the tension that could inhibit milk let-down. Or try a glass of wine or beer occasionally to enhance let down. Warm breast soaks or a warm shower priod to nursing sessions may help. When baby goes on, take four or five deep breaths and slowly exhale to relievethe initial tenderness.
Check your nipple shape when baby comes off the breast. If it always looks squashed, see expert help"
I am also going to add on to what the book says, by saying that it might be beneficial for your sister to invest in some nipple shields. They are relatively inexpensive, and will be a great help in protecting her nipples.
I hope she starts to feel better soon!