Hi, i'm not a member of Weight Watchers, but I know a little bit about it, aswell as Scottish Slimmers, which is similar. You might know this already, but I think each point is worth around 50 calories, which means that your neice should be consuming up to 1650 calories a day, in order to lose a healthy amount of weight (up to around 2lb) per week. As she loses weight, she will need to decrease the number of points she consumes, in order to keep losing. For example, the Scottish Slimmers plan uses points that are worth 25 calories each. If someone was between 10 and 13 stone, they should aim to eat up to 45 points a day (1125 calories), if they get below 10 stone, they must reduce their calorie intake by 5 points, which leaves 40 points, or 1000 calories.
Like I said, I don't know much, but I hope this helps. I'm glad that your neice is no longer starving herself, and that she has such a caring relative.