Another way to establish limits is to set clear expectations with everyone involved, especially the person with cancer. What can she expect your help with, and what's beyond the scope of what you can provide? Explain that while you're going to be the "point person" for caregiving, others will help you make sure everything gets done.
Using the list you made together of all the things she needs help with, focus on working together to assemble a reliable team of helpers to get it all accomplished. If you have siblings, set up a communications system that enables you to delegate tasks to them. Even those who live at a distance can take on a set of obligations. For example, you might give your sister on the opposite coast the job of dealing with medical insurance, or your brother could take over financial planning questions.
If you don't know all of your loved one's friends and neighbors, don't be shy about asking. Remind her about her bowling league, her church community, and any support network available, and ask how to get in touch with these folks.
Friends, neighbors, and other members' of her community will ask how they can help, and when they do, suggest that they pitch in with cooking, cleaning, driving, and other household needs -- then choose a specific task and assign it.
