I'm guessing because, it's still their property. But also it's maybe because they wouldn't want her to go to the ball.
I'm retarded with these answers.

They were jealous of her and they wanted to find excuses to get mad at her and make her misrable. They wanted every reason for her not to go to the ball because they knew she was more beautiful.

Because they don't want her to outshine themselves in anything, most of all in the presence of the Prince. They are mean, cruel, spoiled girls who were raised to hate and bully anyone who is better than them.

UGH SAME!!!!!

I have so many somethings I don't get it's ridiculous

Because they're selfish. Even when people don't want something, they tend to get possessive as soon as they see that someone ELSE wants it. It's human nature. It happens with children, dogs, cats, etc. all the time. They have a toy they never want to play with, then as soon as they see someone else pick it up, suddenly they want it back.