Is it wrong that I'm going to intentionally make my sister wear an unflattering bridesmaid dress?
What is the purpose of making your bridesmaid wear and ugly dress?
My sister has a rather simple taste in fashion, and I want the people to laugh at her.
This is your wedding, but forcing your sister to wear a horrid dress out of spite is just using your own power to impose on her. Leave your sister to make her own decisions about what she wears.
My sister is a bitch.
It's wrong to try to embarrass your sister by having her wear an unattractive dress. Everyone will know that you picked it out for her and will know that you did it on purpose.