Totally get that.
Is her daughter coming along to the US as a K-1? You can either try to delay the interview by contacting the consulate directly or.....depending on the length of the treatment plan, you can delay departure because the visa will be valid for six months from the medical exam. You could potentially drag things out this way by 8 or 9 months, depending on how busy Spain is for K-1 interviews.
Have you guys received 'packet 3' yet? I'm not sure about Spain specifically, but it usually is sent to the petitioner's email and contains info for the beneficiary related to scheduling the medical, interview, and a list of paperwork the consulate wants.
Btw, oh hey there, neighbor!
I work for Fred Hutch. We actually invented the bone marrow transplant. Seattle is an excellent US city to live in if you need cancer care, including follow up. 🙂