Why does my parents spank me, and not my sister?

first of all i don't really get why my parents spank me most of the time, even though im 17. since what i remember they've been spanking me since i was 6. they usually use plastic hangers, the buckle of the belt, wooden hangers, or a wooden stick. most of the time it leaves bruises or the part they spanked on swells up.(swells takes days to come off) i just don't get why they never spanked my younger sister, shes 12. even though she does the worst things ever. all my parents do is talk. they don't even ground her or something like that. i never saw them spanked her the way they do to me. i just don't get it. i asked them why sometimes, all they told me that its the past and they don't do it anymore more. if they really don't spank anymore..why do they still do it to me until now? i really need some advise ... i'm losing it....