Well, the good news is that the first day you diet, you tend to lose a pound or two just in "water weight." So depending on what your sister weighs, she might be able to drop a pound. But losing 4 pounds in less than a week could be tough. I have two suggestions: 1. a total day long fast. This is really hard but it could work for this week. Or 2. reduce calories to about 1000 a day and exercise pretty hard: running, walking, treadmill, biking, swimming, etc. The exercise will help tone her so clothes may fit better even if she only loses about 2 pounds. Plus, she'll feel better on her vacation.

also she should drink lots of cold water while exercising

Healthy diet, supplements may help with weight loss.
Use green tea Spinach and green vegetables.
Enova oil can loose your fat.
Skip all the junk food to vegetables and fruits and eat more protein, drink more water.
I think will best.
http://www.thebeautyinsiders.com/revitashape.html

Visit http://www.nyccosmeticdermatology.com/lipo-zeltiq.htm for a no surgery fat off. You may consider Coolsculpting by ZELTIQ for a fast way to get rid of fats.