Do I really have to take my sister to prom? Probably I'm gonna refuse her.
What do you want to refuse to take your sister to the prom?
I hate her, she is ugly
It would be considered creepy to take a sibling to the prom with you. If you're not comfortable escorting her to the prom, be up front and firm about it.
If she cared about her self esteem, she would've listened to me in the first place. Emotional blackmail is effective but I'm not surprised.
I understand your situation as it's unusual to on a prom with your sister. Apparently you're not comfortable so you should tell that to her.