Sorry you are in this situation, agree it should be possible to do something. The first people to talk to are her doctors, they will advise on the most important things - is she fit to travel (eg is there a risk an airline will refuse to let her on), are there any special arrangements or plans that could or should be made, timescales (eg palliative treatment x first then go after, or, go asap as she may not tolerate it later). They or possibly nursing staff could also talk her through what the risks are, what could happen, and is it likely. You do need to have at least some back-up plan for anything that has a significant risk of happening on holiday. Know what emergency care could be accessed and how if it were needed.
Travel insurance is a secondary consideration - you get insurance if you can for risks that you need cover for (eg cancellation if she gets unwell the day before you go), and take the risks that you can tolerate and/or can't get affordable insurance for (eg worst happens and she deteriorates on holiday, would you all just shell out to get her back to the UK, or treatment where she was, or whatever).
My parents managed to visit us all at Christmas, from Australia. Both had major health issues and I'm not sure they tried to get any insurance in the end. My dad was terminal and unfortunately they had to shorten the originally planned 3 months but still the time they had with my sister and I and grandchildren was worth it. Part of their planning was having money available if treatment not covered by reciprocal arrangements had been necessary. "Repatriation" expenses were irrelevant as if either had been unable to travel home they would have just stayed in the UK. The one big worry was minimising the risk of either of them becoming unwell during the flight and potentially getting stranded in the middle east. Travelling business class helped a lot. (far less of an issue for short haul!)
I hope you and your family can arrange something that will work for all of you and that it is a really memorable holiday when it comes.