I've lived with eczema my entire life. When I was a kid, it was behind my ears, and then it was behind my legs, and then it was on my arms. Weird how it moves around the body as I got older. The docs always said it would get better when I reached adulthood, and I suppose it has. I'm almost 22 now, and I still have mild eczema in the nook of my arms sometimes, though now the primary locations that actually bother me are my wrists and knuckles around my pinky fingers, though not very seriously.
How old is your sister? The worst times, if I recall correctly, were in the middle school years. When it flares up it's almost impossible to be comfortable anywhere.
Tell her not to use water that's too hot when she bathes, because that will strip the oil off of the skin and dry skin = irritation. I SWEAR by Aveeno daily moisturizing lotion - I've tried almost every lotion available over the years, and nothing beats it. Don't use soaps that dry your skin like Ivory - Stick with soaps like Dove, or moisturizing body washes. Stuff like Neutrogena Pore Refining Cleanser and Clean and Clear Cold Cream Cleanser work well for the face, though thankfully I've never suffered from eczema there. The constant dryness has one good thing about it, I suppose - dry skin = little to no acne.
Anyways, one remedy I used when it was going crazy was using an icebag and placing it on the area that was causing a lot of trouble. Putting cold water on a towel, wringing it, and placing it on affected areas also helped a little. Creams and such didn't work so well, since I'd usually scratch the area unconsciously before it would really take effect. Gold Bond medicated powder, or some kind of baby powder with menthol also alleviates some of the discomfort. I'm also a fan of shoulder and neck massage, which helps take the mind off the discomfort.
I hope your sister gets better. Funny how everyone takes their skin for granted until it goes insane.
Oh, and if any areas are severely dry/cracked, in addition to the Aveeno, buy some Aquaphor and apply it to the megadry areas. It helps maintain moisture and really does help healing, though it is an ointment so there will be a little bit of greasy discomfort.