Cabin in the Rockies

This is the cabin my sister and her husband have been doing extensive remodeling on in order to live there year round.  Originally his family's summer cabin, it has been the source of many family get-togethers and many happy memories. 
They lifted the ceiling, insulated inside and underneath the house, knocked out walls of small rooms to make an open floor plan, and have rewired, replumbed and refinished every inch of it.  It has a beautiful new bathroom (in the former days it had an outhouse.  I remember that from 1968.)  It has a new laundry set-up and lovely new windows.  The kitchen is tiny but charming.
This is 700 square feet of space.  How would you reduce the accumulation of 42 years of marriage into 700 square feet?  The choices of what to keep and what to part with are mind boggling.  Of course there are family mementos from earlier generations but what  do you have room to carry with you?
I'm afraid the process would kill me.  I am such a collector 1) because I love everything and am sentimental and 2) because I can think of so many ways to use my stuff when I am called upon to decorate a conference or banquet...But mostly, I just store it.  I admire my sister for the philosophical way she is paring down in order to live the way they want, in a cabin among the aspens.
By the way, the pictures on the wall are photos taken by my brother-in-law who is very talented in photography.