Before you make any real changes to the design, think about your sister's long-term response. Will she push a bit, then see your logic as positive? Will she give into the changes, but regret it later? She's your sister. You know better than anyone else how she will react to being "talked into" something she didn't want. I highly recommend you go through some of the coop pages here with her. There are some really great ideas on BYC. You'll likely end up cobbling pieces together for a wonderful coop and run. Just please, document all of your steps so we can follow the progress. It's so cool to watch things grow!