I think you are totally valid in wanting to choose your own wedding photographer - it is one of the most important vendors for your wedding day and it's what you'll have to remember it by years and years later. From a photographer's perspective, I've definitely seen both sides of the spectrum - photographers who love photographing their loved one's weddings and photographers who refuse to photograph them because it can be hard to maintain a professional relationship and boundaries. I would recommend you telling her that you want her to enjoy the wedding and how much you value her being there for you as a bridesmaid. A bridesmaids' duties are too much to also cover the photographer's job - besides just wedding party photos, there's getting ready, the ceremony, and toasts. Don't feel bad about not choosing her!