From what I have read some people have trouble eating all the food offered on the meal plan. Some are paying for strangers meals and such with their left overs. I see no reason why you could not share your meals with your sister.
That being said, IMHO, I can see Disney trying to close that loop hole in the future. They lose out on revenue with people using their leftovers on other diners.
I can just imagine how magical it must feel to be one of those lucky diners though. Gives me a warm feeling just thinking about it.
Slightly Goofy/Linda