I solved the problem by going off to college. However, I was still called when my Mom had an emergency, despite my being the child and her being the adult. That being said - my Mom has really been here for me since I got cancer.

I think the sooner you set boundaries, the better. Until I left home, I thought it was my duty to help care for my younger brother. Then when I went to college, I had too much stuff I needed to focus on for myself - and to this day my brother resents me for this. He never understood that my purpose in life was not to make his life easier, and because he is so spoiled, his friendships are very superficial. I have fewer friends, but they are deep, meaningful ones. Often with other older children!