Thanks so much everyone, there has been some great advice and I feel much better about the whole situation. I emailed my cousin and told her that I would need to decide on things closer to the date, and left it at that.
My charitable interpretation is that my cousin is overwhelmed by the planning and wasn't intending to be cruel, just didn't think how her email would come across. I get the feeling that she is under a lot of weird pressure from her mother's side of the family, but as we are obviously not that close I can't say for sure. As for needing to know about the accommodation so early, she is having a destination wedding of sorts, so I think that people are spot on about her trying to negotiate a deal with a hotel.
Weddings can make people crazy, and I needed a outside check if my hurt feelings were petty and the first sign of me going over the edge or not. So thank-you all so much. A lot of friends and family are making a huge effort to be there for me on the day, which really is humbling, and what I should be focusing on, not minor dramaz.