Sugar may theoretically have some effects on sleep, depending on an individual's sensitivity to it. When you eat a lot of sugar right before bed, your blood sugar climbs high and then falls rapidly as your body releases hormones to bring the levels under control. This swing in hormones and blood sugar levels may impair sleep. In general, it is not a good idea to eat a large meal or a lot of calories (including sugar) right before bed for this reason.
There are other things that can impair sleep even more than sugar. For example, if your sister eats a lot of chocolate, it is probably the caffeine and not the sugar that is the real problem. Caffeine is a stimulant and it impairs falling asleep. It is important to avoid chocolate, tea, coffee, and sodas before bed if you are having trouble falling asleep.
Also, alcohol has major effects on sleep architecture and is one of the principle causes of waking up in the middle of the night after falling asleep.
Your sister should take a look at her eating habits and lifestyle and, if she is still having trouble sleeping after making some basic changes, go see her primary care doctor