Laura --
You're right to think about your mother's decline in advance! I started helping my father run his checkbook long before his failing vision made it necessary; during that time, he came to trust my understanding of his idiosyncratic record-keeping, and he eventually became comfortable giving me control of this important task.
In the early days, I was often frustrated because my father's processes were inefficient and I knew I could do better. WATCH OUT. It's much more important to respect and understand your parent's ways (screwy though they may be) than to do things "correctly." I learned to wait him out, walk through all the steps he wanted me to take, use his vocabulary, and leave things as he wanted them. As a result, I actually learned useful tricks; more importantly, I made him comfortable. (Perhaps most valuable to me, I also gained some real patience, never my strong suit.)
I encourage you to ask your mother to show you how she does something like balancing her checkbook, paying her credit card, keeping the right food in the kitchen, or watering the garden on the right schedule . . . some task that you and/or your sister think you will have to take on in the future. Start slowly, don't tell her what to do, show appreciation for good organization, etc. You can't guess when it will be appropriate to step in and DO things for her, but that will be much easier if she feels that you know the right way to do it -- i.e., you know how she does it.
