There's an excellent book, called
The Unexpected Journey of Caring by Donna Thomson and Zachary White, that's helped me a lot. For one thing, I learned that many caregivers lose the ability to "see" the future. I'd noticed that in myself, and thought it was my own personal weakness. It's actually a survival skill. Caregiving may go on for years or it can end tomorrow; your loved one's health can take a sudden turn and your whole life changes in an instant. Caregiving requires you to focus on the present moment, work on the current situation, and block out everything else.
Your sibling has probably gotten used to being unable to plan for the weekend, and now she's suddenly supposed to plan the rest of her life. She probably needs time to rebuild the self-care skills that she lost while caring for others.