I think a lot of the initial weight loss is stored water. By upping intake of water through increased fruit and veg and drinking more, your body ditches all the water it's been hanging onto. This may be obvious water e.g. bloating round ankles, knees or may be on the tummy area.
I read somewhere recently too that the extra fluid helps the liver to function better too. Or was it that the liver can function better because it's not being overloaded with fatty foods.
I think the 'big people lose weight more quickly' thing is what probably leads a lot of people to fail. They have one huge weight loss then they only drop 1-2lbs a week and lose heart.
Plateaus don't have to happen. They are a result of not taking into account your changing body or slipping back into old ways or trying to overdo it.
E.g. if you cut right down on your food, your body will go into starvation mode and hang onto weight for the 'famine' - hence never going below 14 points.
As you lose weight i.e. every time you go below a stone level, your body needs less food so you have to drop a point.
If you take up exercise your body will become more efficient so you need to gradually increase the amount of exercise. This might happen naturally e.g. if you set aside 20 mins to walk you will find you are walking further or if you walk a route you can do it in less time. So to continue to lose weight, increase the exercise time or distance.
Most fitness regimes don't recommend increasing by more than 10% at a time because the body will just get too tired and you will overdo it one day and then muscle fatigue will mean you don't do anything for the next few days!! There are useful training timetables online for things like the Breast Cancer Moonwalk . It doesn't mean your SIL has to enter, it just gives some idea of a sensible programme.
She could do lightlife or similar to lose the weight quickly but then she will still have to learn the good habits for the future to keep the weight off.
Anyway, after all that - I hope she goes for it. Like your SIL I am 5' 9". In early Oct I weighed 16st 11lb. I have now lost 24lbs, dropped almost two dress sizes from 20-22 to 16-18 and feel so much better. Even little things like getting in and out of the car, off the sofa, squatting down to pick something up have got easier. I've been here before - lost 5.5 stone last time and put it back on. This time it's for keeps, as it's my 40th birthday in November and I am determined not to be Fat and Forty!