I can hear your pain throughout your post, Mandie, and wish I could take it away from you. That's so incredibly hard to witness.
I can share my experience, though our family didn't have to make crucial, frightening decisions the way you and your sister do .... My mother died from lung cancer in October 2005, a couple weeks before her 61st birthday. We knew she had lung cancer but, like you, didn't really see it coming. Four weeks before her death, she seemed to be doing very well under the circumstances. The doctor suggested she get outpatient surgery on a Friday to address a side effect from radiation treatment, which my mom did. She was home that Friday afternoon, ran a fever all weekend, was admitted to the hospital on Monday, was put into ICU on Tuesday, was intubated by Wednesday and lost consciousness by the following weekend. We were stunned as we watched her slowly drift away from us over the next three or four weeks. The medical staff discussed putting my mother into hospice if she proved to be transitioning toward death or in a nursing home for rehab if she proved to be pulling through, but she didn't even make out of the hospital .... No words for it, really.
People handle grief and dying in so many different ways, and it all has to be honored. Some dying people are afraid to be left alone. Others, like my mom, only want to be left alone .....There's no textbook for these things .... The hardest part of the process for my family, besides watching my mother die, was accepting our powerlessness and vulnerability -- - and hers -- at a time when we wanted everything to be in black-and-white and readily defined (to feel we're in control and to override the underlying fear and grief, I guess). IME, all you can do is surrender to it. The process is so much bigger than anyone in the room.