Early Gray Hair Blues
It all boils down to that chemical --melanin -- the very same chemical pigment that makes some of you light-skinned and some of you dark-skinned. In terms of hair, black hair has the most melanin; blond hair the least. And, gray hair, you ask?
The answer is, quite honestly, that there is no such thing.
Your sister may look like she has gray hair, but she doesn't have gray hairs. If you look up close you'll discover that what she has are a mix of dark hairs and white hairs -- and together they look gray from a distance. You get white hairs when individual cells that make the pigment die off and the hairs have absolutely nothing coloring them.
You do inherit things like the tendency to have your hair turn gray from your parents. However, there's no simple way to know whether you inherited the same genes from your parents that your sister did! But, listen, don't worry if you do eventually turn gray. Take it from a worm -- gray is an extremely attractive color!