How can I be everything to everyone and still support my sister?

Today is the day after Thanksgiving. I must preface this post by saying how grateful I feel to be healthy and living here in the US. 

My only living sister was diagnosed with infiltrating intraductal carcinoma with local metastasis (8 of 10 nodes), stage IIIA in July of 2006.  My sister was 59 at the time, never married, no children and living alone 65 miles from my home.  She has a great job, good support in her community, but, like anyone with this dreaded disease, needed, wanted the support of a close family member.  Our younger sister died at age 45 (unrelated to breast ca), our father a year later and our mother is living and well in a community 120 miles from her.  She is also 83 years old.

Me - married for 37 years, 3 grown children all married with beautiful families (we have 6 grandchildren).  I retired last May after 38 years in the healthcare field.

Our history together - our Dad was career military and our family of origin was the only stability we had growing up.  We are only 16 months apart in age. 

My dilemma - it seems impossible at times to meet her expectations as primary care giver/driver/ support person,  while still maintaining my commitment to husband, children, grandchildren, and our mother.

She has had a very rough time for last year and 4 months.  She had three surgeries, very intense chemotherapy and after 6 months of that - 3 months of radiation - every day.  She had like 14 weeks this summer of feeling like, OKAY - we killed the cancer - what's next??  - Well, now she has metastatic lesion on her liver and her oncologist (we call him Dr. McDreary) believes she has recurrent cancer in the breast - while the surgeon and radiation therapist believe it is reaction to radiation.  Yikes.  Keeping this all straight and reporting to our mother, and keeping my husband from "feeling LEFT OUT!" - is almost enough to drive me to drink!

Wednesday this week I took her to a local town for a PET/CT scan.  Had to leave my house early to get there - the scan took 2 1/2 hours.  When we left she wanted to go out to a sit down dinner together.  I tried to explain that I had pies to finish for Thanksgiving and my husband was preparing dinner for me at home.  She was very upset, and thought my husband was "controlling" me.  I finally had to say - every decision I make is my own.  She didn't come to Thanksgiving dinner (in order to preserve her energy) - I called her and sent pictures from the festivities.  Now she is "punishing" me by not responding.

When I tried to share with my husband some of the pain and frustration - he wanted to compare it to his 93 year old mother - who is healthy, but obviously elderly.  He doesn't get it.

I feel better already just sharing this frustration.  I know there are so many who have gone through worse times.  As I said at the beginning - I feel incredibly blessed in this life.   As difficult as it is to enterain the notion that I may live to bury another sister - the thought that I may have to support my mother through losing another child is almost unbearable.