My mom made me do this super unfair punishment for bullying?

I was being really mena to my sister. My sister is a b---- and i was making fun of her a lot and I started a lot of rumors about her. Now my mom says I have to smell her feet every day after track practice for a month. It is so stupid and disgusting and I HATE IT. I talked to my mother and she says she will not change the punishment. My sister loves it. What should I do?