Who is the slimmest of all?

My sister was looking for an inch tape today. Everyone was wondering what she wants to do with it until I appeased their curiosity by giving the correct answer that she wants to measure her waist. T thinks that she is growing fat. With weighing just 42 kg. at the age of 17, how could one think of growing fat? I wonder when would it be the time for me to worry about waist?