Why does my sister want me to take Pictures of her in a Wedding Shower?

Im newer to America but my sister has been here 10 years and shes getting married. She says that brides and brooms hold wedding showers and she is having one. She wants me to take pictures of her during her wedding shower. I feel uncomfortable taking pictures of her in the shower when shes undressed. How can I tell her and the broom that I don't want to see them in the shower and take pictures?