My niece has asked me to attend a purity ball with her but I would rather not.?

My niece is 11 and her father is not in the picture. Her mother saw this special on tv on Vh1 about purity balls and she's convinced my niece that she should have one and now it's all my niece wants. I'm not a Christian (my niece's mother is a devout Christian and she raising my niece the same way) and the ceremony is Christian so I'm not sure anyone would feel comfortable with an ex-Christian attending. Secondly, I researched the purity ball concept and I just can't morally bring myself to remotely support one. My niece only wants to go to one because she wants to dress up for a night and dance and such (which is fine) but I find the concept of a purity ball to by psychologically damaging and really disgusting. I just think it's... unnatural for my niece to pledge her virginity to me until she's married. But I love my niece as much as I would if she were my own daughter and I hate to deny her anything. I've told her mother I feel uncomfortable with it and now she had badmouthed me to my niece and my niece won't speak to me. I feel very much between a rock and a hard place. Are there are any alternatives to a purity ball that I could attend with her, maybe?