Every one loses weight at a different rate and the rate changes also at different times of the month for women. The biggest difference between the diets is i think that with WW and other food diets you have to have acertain amount of control to just stick to the portions/points/sins you are allowed where as with VLCDs that is taken out of the equation, theres no weighing, measuring or counting you just have your packs. I know for me at the moment i cant do a food diet, i love SW think its the best diet out there but i couldnt at this time control myself so took food out of it and im losing nicely. Also after a short while with WW etc the losses slow down to around 1-2 lbs per week so the process of getting slim is much slower but with VLCDs the loss will continue to be around 4-5lbs per week, ww etc dont promise a stone a month do they? So while it may look like your SIL is in the lead you will overtake her very quickly.