engaged! and then there's me

My sister recently got engaged and asked me to take her engagement photos. Unfortunately, she postponed the pictures from 1:30 to 4:30 and then took two and a half hours to get ready. When we finally got outside, there was hardly any light left, so we had to reschedule.
My roommate and I also went out to get some shots. She's been anxious for a while to go out and get some shots since she had some fun ideas. Looking at the 500something pictures, I realized why I'm the one behind the camera. My roommate drives me absolutely nuts. She is mean to me all the time and I take it because I don't want to cause problems, but I seriously wish I could just stand up to her. She thinks she is the best at everything and doesn't hesitate to tell everyone, including snide comments about why you'll never be as good as she is. She is so critical, so judgmental. I don't know if she is insanely jealous or just mean. She likes to tell me why she is better at photography, cooking, makeup, floral design, has more boys who like her, how her missionary never missed a week... Unfortunately, I don't think telling her will help. She will deny it, why can I just take a joke? and in the end the problem will probably get worse.