I did Exante (a VLCD) before coming back to SW, and lost 1.5 stone in about 6 weeks. They are extremely hard diets to follow, so if your sister is sticking to it, then good on her!
I only did it to give my losses a kick start and to get back to a weight where I felt more comfortable to tackle the rest of my weight loss.
Try not to compare your losses to your sister's, she will most probably find losing that weight very difficult, and if she see's you losing weight steadily and eating well, she may well switch when she's had enough of her VLCD.
With VLCD's, as with any diet, you will ONLY put weight back on if you return to your old eating habits. I maintained my weight for the whole of december after coming off my diet, and only put on a few lbs between christmas and new year. All VLCD's promote returning to healthy eating and have a phased re-introduction of food to ensure you are eating a normal, healthy balanced diet, but in reality a lot of people lose weight and return to eating without going through this phase, so never learn the lessons which are available about healthy eating.
It will be hard to see your sister losing weight quickly, but it's not a race. To follow a VLCD I found I had to suspend a lot of normal activities to avoid temptation, whereas on SW you can lose weight healthily and have a normal life while you do.
You will get to your target, it doesn't matter how long it takes!